Method of biotreatment for solid materials in a nonstirred surface bioreactor

ABSTRACT

A method of biotreating a solid material to remove an undesired compound using a nonstirred surface bioreactor is provided. According to the method, the surface of a plurality of coarse substrates is coated with a solid material to be biotreated to form a plurality of coated coarse substrates. The coarse substrates have a particle size greater than about 0.3 cm and the solid material to be biotreated has a particle size less than about 250 μm. A nonstirred surface reactor is then formed by stacking the plurality of coated coarse substrates into a heap or placing the plurality of coated coarse substrates into a tank so that the void volume of the reactor is greater than or equal to about 25%. The solid material is biotreated in the surface bioreactor until the undesired compound in the solid material is degraded to a desired concentration.

[0001] This application is a continuation of co-pending U.S. patentapplication Ser. No. 09/735,156, filed Dec. 12, 2000 by William J. Kohr(Attorney Docket No. 258/212), which is a continuation of Ser. No.09/097,316, filed Jun. 12, 1998 (Attorney Docket No. 233/157), now U.S.Pat. No. 6,159,726, which is a continuation of U.S. patent applicationSer. No. 08/636,117, filed Apr. 22, 1996 (Attorney Docket No. 219/212),now U.S. Pat. No. 5,766,930, which itself is a continuation-in-part ofco-pending U.S. patent application Ser. No. 08/588,589, filed Jan. 18,1996, (Attorney Docket No. 217/275), now U.S. Pat. No. 6,083,730, whichis a continuation-in-part of co-pending U.S. patent application Ser. No.08/459,621 filed Jun. 2, 1995, now abandoned. Each of the foregoingapplications is incorporated herein by reference as if fully set forth.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to the biotreatment of solidmaterials. In particular, the present invention relates to the ex situbiotreatment of solid materials in an aerobic process to degrade anundesired compound present in the solid material.

[0004] 2. Description of the Prior Art

[0005] Biological treatment processes are finding application throughoutindustry. Such processes have been used in waste water treatment,hazardous waste remediation, desulfurization of coal, and biooxidationof refractory sulfide ores.

[0006] A variety of methods can be employed in the biological treatmentof solid materials, including in situ treatment, landfarming,composting, heap treatment, and stirred or agitated tanks. In the exsitu biological treatment of solid materials, some sort of bioreactor isused to carry out the biotreatment. A bioreactor can be defined as avessel or body in which biological reactions are carried out bymicroorganisms, or enzymes they produce, contained within the reactoritself. The main objective in the design of a bioreactor is to generatean optimal environment for the desired biological process to take placeon a large and economic scale.

[0007] When a solid material is being biotreated, the desired biologicalreactions typically involve the degradation, either directly orindirectly, of some undesired compound present in the solid material. Toaccomplish this economically, the bioreactor needs to reduce theconcentration of the undesired compound to an acceptable level in anacceptable quantity (in terms of flow rate) of solid material to betreated.

[0008] In general biotreatment processes are slow, and if they areaerobic, they require large amounts of oxygen for the aerobicmicroorganism(s) to metabolize, either directly or indirectly, theundesired compound. Oxygen transfer, therefore, is typically a majorproblem for the large class of aerobic biological treatment processesavailable. Current aerobic bioreactor designs attempt to ensure not onlythat the microorganisms being used have access to the material to bebiooxidized or metabolized, but also that all areas of the bioreactorhave an adequate oxygen and nutrient supply, as well as maintain thecorrect pH and temperature, for the biological process to proceed.

[0009] Stirred tank bioreactors are used in many types of aerobicbiological processes, including biooxidation of refractory sulfide goldores and bioremediation of contaminated soils. Stirred tank bioreactorsprovide very good contact between the bioleachant and the solid materialto be treated. In addition, stirred tank processes typically havefavorable oxygen conditions because the tank is sparged with air oroxygen. However, even in stirred tank bioreactors where oxygen isprovided by air or oxygen sparging, the low solubility of oxygen inwater (10 ppm) requires a large gas-water interface. This is generallyachieved with impellers and significant expenditures of energy. The highenergy costs associated with stirring and aerating the reactor make thistype of bioreactor primarily applicable to bioprocesses that come to adesired end point relatively quickly, typically less than a week. Forslower biological processes, a low energy cost, large scale, generallystatic batch process, is the best solution. However, the goal ofproviding the bacteria, or other microorganism, with an optimalenvironment is still of primary importance.

[0010] There are three primary types of static batch bioreactors used tobiotreat soils contaminated with toxic organic compounds. One of thesemethods is landfarming. This is an above grade treatment of contaminatedsoil in a large open space. The soil is spread over a high-densitypolyurethane lined area generally covered with sand to allow fordrainage. Air can be introduced by perforated pipes and by tilling thesoil once or twice a week. This method has been widely implemented atsites contaminated with polynuclear aromatic (PNA's) andpentachlorophenol (PCP). One limitation of this process is that a largearea is needed because the soil is spread relatively thinly to ensureadequate air flow. This method also requires tilling and may be limitingin air if the layer of soil is too thick or does not mix well.

[0011] Another technology used in the bioremediation of contaminatedsoil is composting. The compost is made up of contaminated soil andvarious amendments necessary for composting to be sustained such as woodchips, straw, or manure. These amendments increase the amount ofbiodegradable organics, structurally improve the compost matrix byreducing bulk weight and increasing air voids, and increase the amountof inorganic nutrients in the mixture. The composting can be carried outin a vessel with forced air flow or in open piles that are aerated byair pipes or by tilling. One disadvantage to the addition of organicamendments is that their biodegradation generates heat and requiresoxygen. Composting is usually run in batch mode and a portion of thecompost is used to inoculate the next compost. This process has beenused effectively on many types of organic contaminates including dieselfuel, 2,4,6 trinitrotoluene (TNT), polyaromatic hydrocarbons (PAH),benzene, and xylene.

[0012] Heap bioremediation is another static bioprocess used in thebioremediation of excavated contaminated soil. In this process the soilis placed in piles 8 to 12 feet high over a lined area. To improve airflow, air can be introduced by perforated pipes. In such circumstances,the pipes are placed on approximately a 12 inch bed of the contaminatedsoil in regular intervals. The pipes are then typically covered with alayer of gravel to protect them from the heavy equipment. The excavatedsoil is then dumped in an 8 to 12 foot high pile on top of the gravel.Moisture is maintained with an irrigation system. The soil may needfertilizer or lime to adjust pH and may need sand to increase porosity.This process is low cost and thus is applicable to slow biologicalprocesses. However, this process may be too slow if the heap becomes airlimited due to compaction of the soil during or after pile construction.

[0013] Therefore, air and liquid access remain important rate limitingconsiderations in existing static batch bioprocesses used for soilremediation, such as heap pile bioremediation, composting andlandfarming. Air flow is improved in existing processes to the extentpossible by introducing air through perforated air pipes or by tillingthe soil. However, any flow constriction within the bioreactor willinterfere with the efficiency of the process. Also, if parts of thecontaminated soil are not exposed to bacteria or other nutrients as wellas oxygen, the overall bioprocess will be slowed or not proceed tocompletion. Similarly, in the case of heap biooxidation of coal andrefractory sulfide gold ore, biooxidation of the sulfides is efficientlycarried out by the bacteria only when the metal sulfides are exposed tobacteria, water, nutrients, and air. If the sulfides are buried in theore or in the solid pieces of coal, the biooxidation will not proceed.In addition, if air or liquid flow in the heap becomes limited, thebiooxidation will also become limited. Consequently, a need exists foran improved bioreactor design that will permit the biotreatment of solidmaterials with improved air and fluid flow throughout the bioreactor andthe solid material to be treated.

[0014] The use of acidophilic, autotrophic bacteria to biooxidizesulfide minerals in refractory sulfide ores is one biotreatment that hasgained particular vigor in the last ten to twenty years.

[0015] Gold is one of the rarest metals on earth. Gold ores can becategorized into two types: free milling and refractory. Free millingores are those that can be processed by simple gravity techniques ordirect cyanidation. Refractory ores, on the other hand, are not amenableto conventional cyanidation treatment. Gold bearing deposits are deemedrefractory if they cannot be economically processed using conventionalcyanide leaching techniques because insufficient gold is solubilized.Such ores are often refractory because of their excessive content ofmetallic sulfides (e.g., pyrite and arsenopyrite) and/or organiccarbonaceous matter.

[0016] A large number of refractory ores consist of ores with a preciousmetal such as gold occluded in iron sulfide particles or other metalsulfide particles. The iron sulfide particles consist principally ofpyrite and arsenopyrite. Precious metal values are frequently occludedwithin the sulfide mineral. For example, gold often occurs as finelydisseminated sub-microscopic particles within a refractory sulfide hostof pyrite or arsenopyrite. If the gold, or other precious metal, remainsoccluded within the sulfide host, even after grinding, then the sulfidesmust be oxidized to liberate the encapsulated precious metal values andmake them amenable to a leaching agent (or lixiviant); thus, the sulfideoxidation process reduces the refractory nature of the ore.

[0017] A number of processes for oxidizing the sulfide minerals toliberate the precious metal values are well known in the art. Thesemethods can generally be broken down into two types: mill operations andheap operations. Mill operations are typically expensive processeshaving high operating and capital costs. As a result, even though theoverall recovery rate is typically higher for mill type processes, milloperations are typically not applicable to low grade ores, that is oreshaving a gold concentration less than approximately 0.07 oz/ton. Milloperations are even less applicable to ores having a gold concentrationas low as 0.02 oz/ton.

[0018] Two well known methods of oxidizing sulfides in mill typeoperations are pressure oxidation in an autoclave and roasting.

[0019] Oxidation of sulfides in refractory sulfide ores can also beaccomplished using acidophilic, autotrophic microorganisms, such asThiobacillus ferrooxidans, Sulfolobus, Acidianus species andfacultative-thermophilic bacteria in a microbial pretreatment. Thesemicroorganisms utilize the oxidation of sulfide minerals as an energysource during metabolism. During the oxidation process, the foregoingmicroorganisms oxidize the iron sulfide particles to cause thesolubilization of iron as ferric iron, and sulfide, as sulfate ion.

[0020] If the refractory ore being processed is a carbonaceous sulfideore, then additional process steps may be required following microbialpretreatment to prevent preg-robbing of the aurocyanide complex or otherprecious metal-lixiviant complexes by the native carbonaceous matterupon treatment with a lixiviant.

[0021] As used herein, sulfide ore or refractory sulfide ore will beunderstood to also encompass refractory carbonaceous sulfide ores.

[0022] A known method of bioleaching carbonaceous sulfide ores isdisclosed in U.S. Pat. No. 4,729,788, issued Mar. 8, 1988, which ishereby incorporated by reference. According to the disclosed process,thermophilic bacteria, such as Sulfolobus and facultative-thermophilicbacteria, are used to oxidize the sulfide constituents of the ore. Thebioleached ore is then treated with a blanking agent to inhibit thepreg-robbing propensity of the carbonaceous component of the ore. Theprecious metals are then extracted from the ore using a conventionallixiviant of cyanide or thiourea.

[0023] Another known method of bioleaching carbonaceous sulfide ores isdisclosed in U.S. Pat. No. 5,127,942, issued Jul. 7, 1992, which ishereby incorporated by reference. According to this method, the ore issubjected to an oxidative bioleach to oxidize the sulfide component ofthe ore and liberate the precious metal values. The ore is theninoculated with a bacterial consortium in the presence of nutrientstherefor to promote the growth of the bacterial consortium, thebacterial consortium being characterized by the property of deactivatingthe preg-robbing propensity of the carbonaceous matter in the ore. Inother words, the bacterial consortium functions as a biological blankingagent. Following treatment with the microbial consortium capable ofdeactivating the precious-metal-adsorbing carbon, the ore is thenleached with an appropriate lixiviant to cause the dissolution of theprecious metal in the ore.

[0024] Oxidation of refractory sulfide ores using microorganisms, or asit is often referred to, biooxidation, can be accomplished in a millprocess or a heap process. Compared to pressure oxidation and roasting,biooxidation processes are simpler to operate, require less capital, andhave lower operating costs. Indeed, biooxidation is often chosen as theprocess for oxidizing sulfide minerals in refractory sulfide oresbecause it is economically favored over other means to oxidize the ore.However, because of the slower oxidation rates associated withmicroorganisms when compared to chemical and mechanical means to oxidizesulfide refractory ores, biooxidation is often the limiting step in themining process.

[0025] One mill type biooxidation process involves comminution of theore followed by treating a slurry of the ore in a stirred bioreactorwhere microorganisms can use the finely ground sulfides as an energysource. Such a mill process was used on a commercial scale at the TonkinSprings mine. However, the mining industry has generally considered theTonkin Springs biooxidation operation a failure. A second mill typebiooxidation process involves separating the precious metal bearingsulfides from the ore using conventional sulfide concentratingtechnologies, such as floatation, and then oxidizing the sulfides in astirred bioreactor to alleviate their refractory nature. Commercialoperations of this type are in use in Africa, South America andAustralia.

[0026] Biooxidation in a heap process typically entails forming a heapwith crushed refractory sulfide ore particles and then inoculating theheap with a microorganism capable of biooxidizing the sulfide mineralsin the ore. After biooxidation has come to a desired end point, the heapis drained and washed out by repeated flushing. The liberated preciousmetal values are then ready to be leached with a suitable lixiviant.

[0027] Typically precious metal containing ores are leached with cyanidebecause it is the most efficient leachant or lixiviant for the recoveryof the precious metal values from the ore. However, if cyanide is usedas the lixiviant, the heap must first be neutralized.

[0028] Because biooxidation occurs at a low, acidic pH while cyanideprocessing must occur at a high, basic pH, heap biooxidation followed byconventional cyanide processing is inherently a two step process. As aresult, processing options utilizing heap biooxidation must separate thetwo steps of the process. This is conventionally done by separating thesteps temporally. For example, in a heap biooxidation process of arefractory sulfide gold ore, the heap is first biooxidized and thenrinsed, neutralized and treated with cyanide. To accomplish thiseconomically and practically, most heap biooxidation operations use apermanent heap pad in one of several ore on—ore off configurations.

[0029] Of the various biooxidation processes available, heapbiooxidation has the lowest operating and capital costs. This makes heapbiooxidation processes particularly applicable to low grade or wastetype ores, that is ores having a gold (or an equivalent precious metalvalue) concentration of less than about 0.07 oz/ton. Heap biooxidation,however, has very slow kinetics compared to mill biooxidation processes.Heap biooxidation typically requires many months in order to oxidize thesulfide minerals in the ore sufficiently to permit the gold or otherprecious metal values to be recovered in sufficient quantities bysubsequent cyanide leaching for the process to be considered economical.Heap biooxidation operations, therefore, become limited by the length oftime required for sufficient biooxidation to occur to permit theeconomical recovery of gold. The longer the time required forbiooxidation the larger the permanent pad facilities and the larger thenecessary capital investment. At mine sites where the amount of landsuitable for heap pad construction is limited, the size of the permanentpad can become a limiting factor in the amount of ore processed at themine and thus the profitability of the mine. In such circumstances, ratelimiting conditions of the biooxidation process become even moreimportant.

[0030] The rate limiting conditions of the heap biooxidation processinclude inoculant access, nutrient access, air or oxygen access, toxinsbuild up, and carbon dioxide access, which are required to make theprocess more efficient and thus an attractive treatment option.Moreover, for biooxidation, the induction times concerning biooxidants,the growth cycles, the biocide activities, viability of the bacteria andthe like are important considerations because the variables such asaccessibility, particle size, settling, compaction and the like areeconomically irreversible once a heap has been constructed. This isbecause heaps cannot be repaired once formed, except on a limited basis.

[0031] Ores that have a high clay and/or fines content are especiallyproblematic when processing in a heap leaching or heap biooxidationprocess. The reason for this is that the clay and/or fines can migratethrough the heap and plug channels of air and liquid flow, resulting inpuddling; channelling; nutrient-, carbon dioxide-, or oxygen-starving;uneven biooxidant distribution, and the like. As a result, large areasof the heap may be blinded off and ineffectively leached. This is acommon problem in cyanide leaching and has lead to processes of particleagglomeration with cement for high pH cyanide leaching and with polymersfor low pH bioleaching. Polymer agglomerate aids may also be used inhigh pH environments, which are customarily used for leaching theprecious metals, following oxidative bioleaching of the iron sulfides inthe ore.

[0032] Biooxidation of refractory sulfide ores is especially sensitiveto blocked percolation channels by loose clay and fine material becausethe bacteria need large amounts of air or oxygen to grow and biooxidizethe iron sulfide particles in the ore. Air flow is also important todissipate heat generated by the exothermic biooxidation reaction,because excessive heat can kill the growing bacteria in a large, poorlyventilated heap.

[0033] The methods disclosed in U.S. Pat. No. 5,246,486, issued Sep. 21,1993, and U.S. Pat. No. 5,431,717, issued on Jul. 11, 1995 to WilliamKohr, both of which are hereby incorporated by reference, are directedto increasing the efficiency of the heap biooxidation process byensuring good fluid flow (both gas and liquid) throughout the heap.

[0034] Ores that are low in sulfide or pyrite, or ores that are high inacid consuming materials such as calcium carbonate or other carbonates,may also be problematic when processing in a heap biooxidation. Thereason for this is that the acid generated by these low pyrite ores isinsufficient to maintain a low pH and high iron concentration needed forbacteria growth.

[0035] Solution inventory and solution management also pose importantrate limiting considerations for heap biooxidation processes. Thesolution drained from the biooxidation heap will be acidic and containbacteria and ferric ions. Therefore, this solution can be usedadvantageously in the agglomeration of new ore or by recycling it backto the top of the heap. However, toxic and inhibitory materials canbuild up in this off solution. For example, ferric ions, which aregenerally a useful aid in pyrite leaching, are inhibitory to bacteriagrowth when their concentration exceeds about 30 g/L. Other metals thatretard the biooxidation process can also build-up in this solution. Suchmetals that are often found in refractory sulfide ores include arsenic,antimony, cadmium, lead, mercury, and molybdenum. Other toxic metals,biooxidation byproducts, dissolved salts and bacterially producedmaterial can also be inhibitory to the biooxidation rate. When theseinhibitory materials build up in the off solution to a sufficient level,recycling of the off solution becomes detrimental to the rate at whichthe biooxidation process proceeds. Indeed, continued recycling of an offsolution having a sufficient build-up of inhibitory materials will stopthe biooxidation process altogether. The method disclosed in U.S. patentapplication Ser. No. 08/329,002, filed Oct. 25, 1994, by Kohr, et al.,hereby incorporated by reference, teaches a method of treating thebioleachate off solution to minimize the build-up of inhibitorymaterials. As a result, when the bioleachate off solution is recycled tothe top of the heap, the biooxidation rate within the heap is notslowed, or it will be slowed to a lesser degree than if the off solutionwere recycled without treatment.

[0036] While the above methods have improved the rate at which heapbiooxidation processes proceed, heap biooxidation still takes muchlonger than a mill biooxidation process such as a stirred bioreactor.Yet, as pointed out above, with low grade refractory sulfide ores, astirred bioreactor is not a viable alternative due to its high initialcapital cost and high operating costs. A need exists, therefore, for aheap bioleaching technique that can be used to biooxidize precious metalbearing refractory sulfide ores and which provides improved air andfluid flow within the heap. In addition, a need exists for a heapbioleaching process in which ores that are low in sulfide minerals, orores that are high in acid consuming materials such as calciumcarbonate, may be processed.

[0037] A need also exists for a biooxidation process that can be used toliberate occluded precious metals in concentrates of refractory sulfideminerals. Mill processes that are currently used for oxidizing suchconcentrates include bioleaching in a stirred bioreactor, pressureoxidation in an autoclave, and roasting. These mill processes oxidizethe sulfide minerals in the concentrate relatively quickly, therebyliberating the entrapped precious metals. However, unless theconcentrate has a high concentration of gold, it does not economicallyjustify the capital expense or high operating costs associated withthese processes. And, while a mill bioleaching process is the leastexpensive mill process in terms of both the initial capital costs andits operating costs, it still does not justify processing concentrateshaving less than about 0.5 oz. of gold per ton of concentrate, whichtypically requires an ore having a concentration greater than about 0.07oz. of gold per ton. Therefore, a need also exists for a process thatcan be used to biooxidize concentrates of precious metal bearingrefractory sulfide minerals at a rate comparable to a stirred tankbioreactor, but that has capital and operating costs more comparable tothat of a heap bioleaching process.

[0038] In addition to concentrates of precious metal bearing sulfideminerals, there are many sulfide ores that contain metal sulfideminerals that can potentially be treated using a biooxidation process.For example, many copper ores contain copper sulfide minerals. Otherexamples include zinc ores, nickel ores, and uranium ores. Biooxidationcould be used to cause the dissolution of metal values such as copper,zinc, nickel and uranium from concentrates of these ores. The dissolvedmetal values could then be recovered using known techniques such assolvent extraction, iron cementation, and precipitation. However, due tothe sheer volume of the sulfide concentrate formed from sulfide ores, astirred bioreactor would be prohibitively expensive, and standard heapoperations would simply take too long to make it economically feasibleto recover the desired metal values. A need also exists, therefore, foran economical process for biooxidizing concentrates of metal sulfideminerals produced from sulfide ores to thereby cause the dissolution ofthe metal values so that they may be subsequently recovered from thebioleachate solution.

[0039] Therefore, while a need exists for a method of biooxidation thatcan be used to process sulfide concentrates from refractory sulfide oresat a rate which is much faster than that of existing heap biooxidationprocesses, yet which has initial capital costs and operating costs lessthan that of a stirred bioreactor, this need has gone unfulfilled.Further, while a need has also existed for a method of biooxidation thatcan be used to economically process sulfide concentrates of metalsulfide type ores, this need has also gone unfulfilled.

SUMMARY OF INVENTION

[0040] The present invention is directed to the biotreatment of solidmaterials in a nonstirred bioreactor. To this end, in a first aspect ofthe present invention, a method of biotreating a solid material toremove an undesired compound using a nonstirred surface bioreactor isprovided. According to the method the surface of a plurality of coarsesubstrates is coated with a solid material to be biotreated to form aplurality of coated coarse substrates. A nonstirred surface reactor isthen formed by stacking the plurality of coated coarse substrates into aheap or placing the plurality of coated coarse substrates into a tank sothat the void volume of the reactor is greater than or equal to about25%. The reactor is inoculated with a microorganism capable of degradingthe undesired compound in the solid material, and the solid material isthen biotreated in the surface bioreactor until the undesired compoundin the solid material is degraded to a desired concentration. To ensureadequate void volume in the bioreactor, the coarse substrates preferablyhave a particle size greater than about 0.3 cm and the solid material tobe biotreated preferably has a particle size less than about 250 μm. Thethickness of the solid material coating on the plurality of coarsesubstrates is preferably less than about 1 mm to ensure that themicroorganism being used in the biotreatment have adequate access to allof the solid material being biotreated. Thicker coatings will increasethe capacity of the bioreactor, but the rate at which the biotreatmentprocess advances will be slowed due to the limited access of themicroorganism being used to the underlying particles of solid material.To make full use of the capacity of the bioreactor while ensuringadequate microorganism access, the thickness of the solid materialcoating should be greater than about 0.5 mm and less than about 1 mm.For enhanced air and liquid access, the void volume of the bioreactorcan be set to greater than or equal to about 35%. This will greatlyimprove the rate at which the biotreatment process proceeds.

[0041] A variety of materials can be used for the coarse substrates,including rock, gravel, lava rock, barren rock containing carbonateminerals, brick, cinder block, slag, and plastic.

[0042] The process according to the first aspect of the invention isuseful for many different biotreatment processes, including thebioremediation of contaminated soils, the desulfurization of coal, andthe biooxidation of refractory sulfide ores. In bioremediationapplications, the undesired compound is typically an organic compound.In coal desulfurization and refractory sulfide ore biooxidationapplications, the undesired compound is sulfide minerals.

[0043] In a second aspect of the present invention, a method ofbiooxidizing sulfide minerals using a nonstirred surface bioreactor toliberate metal values of interest is provided. The method comprisesobtaining a concentrate of metal sulfide particles from the sulfide orebody to be biooxidized and then coating the concentrate of metal sulfideparticles onto a plurality of substrates, such as coarse ore particles,lava rock, gravel, or rock containing carbonate minerals as a source ofCO₂ for the bacteria. After the metal sulfide particles are coated orspread onto the plurality of substrates, a heap is formed with thecoated substrates or the coated substrates are placed within a tank. Themetal sulfide particles on the surface of the plurality of coatedsubstrates are then biooxidized to liberate the metal values ofinterest.

[0044] Depending on the particular ore deposit being mined, the sulfidemineral concentrates used in this invention may comprise sulfideconcentrates from precious metal bearing refractory sulfide ores or theymay comprise sulfide concentrates from base metal sulfide type ores,such as chalcopyrite, millerite or sphalorite. The distinction beingthat in the former, the metal of interest is a precious metal occludedwithin the sulfide minerals, and in the latter, the metal to berecovered is a base metal such as copper, nickel, or zinc and is presentas a metal sulfide in the sulfide concentrate.

[0045] In a third aspect of the present invention, a method ofrecovering precious metal values from precious metal bearing refractorysulfide ore using a nonstirred surface bioreactor is provided. Themethod according to this aspect of the invention comprises the steps ofproducing a concentrate of metal sulfide particles from the refractorysulfide ore, coating the surface of a plurality of substrates with theconcentrate of metal sulfide particles, forming a heap using theplurality of coated substrates, biooxidizing the metal sulfide particleson the surface of the plurality of substrates, contacting thebiooxidized metal sulfide particles with a precious metal lixiviant tothereby dissolve precious metal values from the biooxidized metalsulfide particles, and recovering precious metal values from thelixiviant.

[0046] According to a fourth aspect of the present invention, a methodof recovering precious metal values from precious metal bearingrefractory sulfide ore using a nonstirred surface bioreactor isprovided. The method according to this aspect of the invention comprisesthe steps of producing a concentrate of metal sulfide particles from aprecious metal bearing refractory sulfide ore, coating the surface of aplurality of coarse substrates with the concentrate of metal sulfideparticles, placing the plurality of coated substrates in a tank,biooxidizing the metal sulfide particles on the surface of the pluralityof coarse substrates, contacting the biooxidized metal sulfide particleswith a precious metal lixiviant to thereby dissolve precious metalvalues from the biooxidized metal sulfide particles, and recoveringprecious metal values from the lixiviant.

[0047] According to a fifth aspect of the present invention, a methodfor recovering metal values from a sulfide mineral ore using anonstirred surface bioreactor is provided. The method according to thisaspect of the invention comprises the steps of: producing a concentrateof metal sulfide particles from the sulfide mineral ore, coating thesurface of a plurality of coarse substrates with the concentrate ofmetal sulfide particles, forming a heap with the plurality of coatedsubstrates or placing the coated substrates into a tank, biooxidizingthe metal sulfide particles on the surface of the plurality of coarsesubstrates to thereby cause the production of a bioleachate offsolution, recovering the desired metal values from the bioleachate offsolution. Ores of particular interest that can be processed using thisprocess include sulfide ores of copper, zinc, nickel, molybdenum,cobalt, and uranium.

[0048] The above and other objects, features and advantages will becomeapparent to those skilled in the art from the following description ofthe preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

[0049]FIG. 1 is a schematic illustration of a process flow chartaccording to one embodiment of the present invention;

[0050]FIG. 2 is a cross sectional view of a refractory sulfide oresubstrate coated with a concentrate of metal sulfide particles inaccordance with the present invention;

[0051]FIG. 3 is a schematic illustration of a process flow chartaccording to another embodiment of the present invention;

[0052]FIG. 4 is a schematic illustration of a process flow chartaccording to yet another embodiment of the present invention;

[0053]FIG. 5 is a schematic illustration of a process flow chartaccording to yet another embodiment of the present invention;

[0054]FIG. 6 is a graph illustrating the percent of iron oxidationversus time for a whole ore compared to a process according to thepresent invention;

[0055]FIG. 7 is a graph comparing the average daily biooxidation rate ofa whole ore against that of a process according to the presentinvention;

[0056]FIG. 8 is a graph illustrating the percentage of biooxidation foranother process according to the present invention;

[0057]FIG. 9 is a graph illustrating the average daily rate ofbiooxidation for the process corresponding to FIG. 8; and

[0058]FIG. 10 is a graph illustrating the percentage of biooxidation asa function of time for a pyrite concentrate coated on a barren rocksupport and the same pyrite concentrate coated on a refractory sulfideore support that contains a high concentration of mineral carbonate.

DETAILED DESCRIPTION OF THE INVENTION

[0059] A first embodiment of the invention is now described in which asolid material is biotreated in a nonstirred surface bioreactor in orderto remove an undesired compound. According to the first embodiment, thesurface of a plurality of coarse substrates having a particle sizegreater than about 0.3 cm is coated with the solid material to bebiotreated to form a plurality of coated coarse substrates. The solidmaterial to be biotreated has a particle size of less than about 250 μmso that it forms a fairly uniform coating on the coarse substrates. Anonstirred surface reactor is then formed by stacking the plurality ofcoated coarse substrates into a heap or placing the plurality of coatedcoarse substrates into a tank so that the void volume of the reactor isgreater than or equal to about 25%. The reactor is inoculated with amicroorganism capable of degrading the undesired compound in the solidmaterial, and the solid material is then biotreated in the surfacebioreactor until the undesired compound in the solid material isdegraded to a desired concentration.

[0060] The biotreatment process can be used in the bioremediation ofcontaminated soils, the desulfurization of coal, and the biooxidation ofrefractory sulfide ores to name a few. In bioremediation applications,the solid material is typically soil and the undesired compound istypically an organic compound within the soil. The present invention,therefore, has application at many of the existing superfund sites. Apartial list of the organic contaminants which can be removed from soilusing the present invention include: waste oil, grease, jet fuel, dieselfuel, crude oil, benzene, toluene, ethylbenzene, xylene, polyaromatichydrocarbons (PAH), polynuclear aromatics (PNAs), pentachlorophenol(PCP), polychlorinated biphenyls (PCBs), creosote, pesticides,2,4,6,-trinitrotoluene (TNT), hexahydro-1,3,5-trinitro-1,3,5-triazine(RDX), octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX),N-methyl-N-2,4,6-tetranitroaniline, and nitrocellulose (NC).

[0061] If, on the other hand, the present invention is used todesulfurize coal, the solid material will be comprised of coal particlesand the undesired compound will be the sulfide mineral particlescontained within the coal particles. In refractory sulfide orebiooxidation applications, the solid material will typically be groundore or a sulfide concentrate produced from the ore and the undesiredcompound will be the metal sulfide particles within the ore orconcentrate.

[0062] In some instances it may be beneficial to form a concentrate byflotation or by other means where by the fraction of the solid materialto be biotreated is concentrated in a smaller weight fraction. Thisconcentrate, if it contains the majority of the undesired metal sulfidesor toxins, for example, can be processed more cost effectively than theentire material.

[0063] As those skilled in the art will appreciate from the foregoingand the ensuing description, the process according to the presentinvention has broad applicability in that it can be used to biotreat anysolid material that contains an undesired compound which is susceptibleto biodegradation or biooxidation by a microorganism or the enzymesproduced by a microorganism.

[0064] The purpose of the coarse substrates is to provide a support witha relatively large surface area upon which the solid material to bebiotreated can reside during the biotreatment process. Therefore, when alarge number of coated coarse substrates are stacked in a heap or placedin a tank, a nonstirred surface reactor is formed that has a very largeactive surface area per cubic meter of reactor space. Although the exactsurface area of the reactor per cubic meter of reactor space will dependon the particular size of the coarse substrates employed, it should beat least 100 square meters per cubic meter of reactor and will typicallybe 500 square meters or more per cubic meter of reactor space.Furthermore, by using coarse substrates that have a particle sizegreater than about 0.3 cm and restricting the particle size of the solidmaterial to be biotreated to less than about 250 μm, the reactor will beensured adequate void volume to permit air and nutrients to access allparts of the reactor during the biotreatment process. In this regard,the void volume of the reactor should be at least about 25%. Such a voidvolume will also ensure adequate heat dissipation within the heap. Forenhanced air and liquid access and heat dissipation, the void volume ofthe bioreactor can be set to greater than or equal to about 35%. Thiswill greatly improve the rate at which the biotreatment processproceeds.

[0065] While using larger coarse substrates will increase the voidvolume in the reactor and thus improve air and nutrient access, inaddition to heat dissipation, throughout the entire reactor, the use oflarger substrates reduces the loading capacity of the bioreactor. A goodcompromise between ensuring adequate void volume and ensuring adequatereactor capacity can be achieved by using coarse substrates having anominal particle size that is greater than about 0.6 cm and less thanabout 2.54 cm.

[0066] A variety of materials can be used for the coarse substrates,including rock, gravel, lava rock, barren rock containing carbonateminerals, brick, cinder block, slag, and plastic. Lava rock isparticularly preferred because of its rough, nonuniform surface, thusincreasing its surface area for a given particle size substrate andimproving the integrity of the coating of solid material which isapplied to it. Coarse barren rock containing carbonate minerals isadvantageous if the biotreatment process is acidic because the acid willreact with the carbonate minerals to slowly cause the release of carbondioxide, which autotrophic microorganisms can use as a source of carbonto carry out metabolic synthesis. The carbon dioxide production can thusbe used to promote microorganism growth in the reactor.

[0067] When a refractory sulfide ore or sulfide concentrate is beingbiooxidized to reduce the sulfide mineral content therein, coarse oreparticles can be used as the coarse substrates. Similarly, if theprocess is being used to desulfurize coal, coarse coal particles can beused as the coarse substrates. In both cases, the substrate may benefitfrom the biooxidation process carried out on its surface.

[0068] While the coarse substrates have been defined as having aparticle size of greater than about 0.3 cm, it is recognized andcontemplated that some of the coarse substrate material may actually besmaller than this. As those skilled in the art will recognize that ifthe coarse substrates are produced by crushing lager material to thedesired size range, the crushed material will have a certain sizedistribution. And, even if the material is screened to exclude materialless than about 0.3 cm, some material having a particle size less thanthe 0.3 cm target minimum will still be present in the coarse substratesdue to inherent inefficiencies in the screening process and due toparticle attrition during handling. Thus, by greater than about 0.3 cmit is intended that substantially all of the coarse substrates are abovethis size so that the void volume of the reactor remains above at leastabout 25% during formation of the reactor and throughout its operation.Preferably the amount of coarse substrates below the 0.3 cm cutoff isless than 5% by weight.

[0069] In general, the solid material to be biotreated should be muchsmaller than the coarse substrate onto which it is coated. This materialshould be ground to a small enough size to allow the microorganismemployed in the biotreatment to have access to all the material so thatthe undesired compound can be biooxidized or biodegraded in a time thatis generally larger than a stirred tank process, but shorter than a heapprocess of the whole material. This time will generally be between 14days and 90 days, depending on the undesired compound and the rate ofits biodegradation or biooxidation.

[0070] The maximum solid material particle size has been set at about250 μm so that the solid material will form a relatively uniform coatingon the coarse substrates during the coating process, rather than formingagglomerates between themselves. Furthermore, particles larger than 250μm may not adhere to the surface of the coarse substrates very wellwithout the use of a binder.

[0071] It is desirable to form a relatively uniform coating of the fineparticles on the coarse substrates during the coating process becausethis will maximize the integrity of the coating and the surface area ofthe solid material exposed to the active microorganism which is added tothe bioreactor. If agglomerates of the solid material are formed duringthe coating process, the particles of solid material which are in theinterior of the agglomerate will be blocked from the action of themicroorganism and thus the amount of biological treatment they willreceive will be reduced or nonexistent. Further, the agglomerates arenot as structurally sound as the coated substrates and are likely tobreak apart during the stacking process used to form the reactor orduring biotreatment, potentially leading to the formation of blockageswithin the reactor, which could blind off portions of the reactor fromthe biological treatment.

[0072] Typically as the particle size of the solid material to bebiotreated decreases, the biotreatment process will proceed faster andmore solid material can be loaded onto the coarse substrates. Smallerparticle sizes will also tend to stick better to the surface of thecoarse substrates. If the particle size of the solid material to betreated is less than about 25 μm, however, excessive dust problems couldbe encountered during handling and some clumping may be experiencedduring the coating process.

[0073] Preferably the particle size of the solid material to be treatedhas a nominal particle size which is greater than about 75 μm and lessthan about 106 μm. Particles in this size range will adhere well to thecoarse substrates, and the incremental improvements which can beachieved in the rate of the biotreatment process with finer particlesizes are rarely justified by the added grinding costs of producingthem.

[0074] The coated substrates can be produced by adding the coarsesubstrates and solid material to a rotating drum in appropriatequantities. Preferably the coarse substrates are dry and the solidmaterial is in a high pulp density slurry so that it will stick to thecoarse substrates as the slurry coats the coarse substrates.Alternatively, both the coarse substrates and solid material can be drywhen added to the rotating drum and water sprayed into the drum topromote adhesion of the solid material to the coarse substrates. Informing the coated substrates, it is desirable to maintain the moisturecontent of the solid material within the range of 5 to 30 weight % topromote proper adhesion between the solid material and coarsesubstrates.

[0075] As those skilled in the art will recognize many other techniquescan also be used to coat the coarse substrates. For example, the solidmaterial to be biotreated can be sprayed in a high pulp density slurryform onto the coarse substrates as the plurality of coarse substratesare being stacked to form the reactor.

[0076] If the solid material to be biotreated is applied as a slurry,adjustments can be made to the material to optimize the biotreatmentprocess. For example, the pH can be adjusted to the optimum pH range forthe microorganism that is to be used to break down the undesiredcompound. If nutrients, amendments, or inoculants are needed, they canalso be added at this time. In some cases it may be advantageous tostart the bioprocess in a tank prior to application of the particles ofsolid material to the coarse substrates.

[0077] The integrity of the coated coarse substrates should besufficient enough to prevent a large number of blockages from forming inthe flow channels of the reactor while the particles of solid materialon the surface of the coated substrates are being biotreated. Suchblockages will decrease oxygen flow and microorganism migration withinthe bioreactor and thereby reduce the rate of the biotreatment process.Of course, the larger the coarse substrates are in relation to theparticle size of the solid material, the less likely such blockages willform because the solid material will be much smaller than theinterstices between the coarse substrates. The integrity of the coatedsubstrates should also be sufficient enough to prevent excessive amountsof the solid material from washing from the bioreactor during thebiotreatment process.

[0078] Although the surface tension of water should hold the particlesof solid material to the surface of the coarse substrates in mostinstances, if it is found that the particles of solid material arewashing from the bioreactor in excessive concentrations or thatblockages are forming in the bioreactor due to degradation of thecoating, a binding agent can be used to improve the integrity of thecoating. However, binding agents may interfere with the access of thebiotreatment microorganism to some of the solid material to bebiotreated, thus increasing the time necessary for the biotreatmentprocess to reach the desired end point.

[0079] The thickness of the solid material coating on the plurality ofcoarse substrates is preferably less than about 1 mm to ensure that themicroorganism being used in the biotreatment have adequate access to allof the solid material being biotreated. Thicker coatings will increasethe capacity of the bioreactor, but the rate at which the biotreatmentprocess advances will be slowed due to the limited access of themicroorganism being used to the underlying particles of solid material.To make full use of the capacity of the bioreactor while ensuringadequate microorganism access, the thickness of the solid materialcoating should be greater than about 0.5 mm and less than about 1 mm.When a rock or brick substrate is being used, this will translate into asolid material loading of approximately 10 to 30 percent by weight.

[0080] The nonstirred surface reactor is formed by stacking a pluralityof the coated substrates in a heap or in a tank. Conveyor stacking willminimize compaction of the coated substrates within the reactor.However, other means of stacking may be employed.

[0081] Preferably the reactor is inoculated with the microorganism(s)which is to be used in the biotreatment process while the plurality ofcoated substrates are being stacked to form the nonstirred surfacereactor or immediately after formation of the reactor. Alternatively, ifthe microorganism(s) to be employed in the biotreatment process functionbest in a particular pH range, the pH of the reactor can be adjustedprior to inoculation as is well known in the art.

[0082] The microorganisms which are useful in the present biotreatmentprocess are the same microorganisms that have traditionally been used todegrade a particular undesired compound in existing biodegradation andbiooxidation processes. For example, acidophilic, autotrophic bacteriasuch as Thiobacillus ferrooxidans, Leptospirillum ferrooxidans, andSulfolobus, can be used to biooxidize sulfide minerals in coaldesulfurization or refractory sulfide ore biooxidation applications.Other bacteria that are useful in these applications are well within theordinary skill of those in the art. Similarly, with respect to soilremediation applications, the microorganism(s) which should be employedare the same as those currently employed in present bioremediationprocesses such as composting, landfarming, slurry biodegradation, andheap pile bioremediation. Those having ordinary skill in the art will bereadily able to determine which microorganism(s) are applicable for thevarious undesired compounds which may be removed from the solid materialusing the process according to the present invention.

[0083] Once the reactor is inoculated with an appropriate microorganism,the conditions such as pH, temperature, nutrient supply, and moisturecontent within the reactor should be monitored and maintained throughoutthe biotreatment so as to promote the growth of the microorganism to thefullest extent possible. As the microorganism grows throughout thereactor, the reactor is transformed into a bioreactor having a verylarge surface area that will biodegrade or biooxidize the undesiredcompound in a time much shorter than that of traditional static batchbiotreatment processes such as heap bioleaching, composting, andlandfarming.

[0084] The reactor can also be provided with perforated air pipesthrough which air can be blown or drawn as is well known in the art.Whether air is blown or drawn through the reactor will depend on thespecific bioprocess occurring within the reactor, and such a selectionis also well within the skill of those in the art.

[0085] The biotreatment process should be permitted to proceed until theundesired compound in the solid material is degraded to a desiredconcentration. In the case of soil remediation applications, this willtypically be dictated by governmental regulations which define theacceptable level of a particular contaminant. In coal desulfurizationapplications, the amount of residual sulfur which is permitted to remainin the coal will also depend, to a large extent, on environmentalregulations, because when sulfur bearing coal is burned it will producesulfur dioxide as a byproduct. Thus, the amount of sulfur allowed toremain in the coal should be less than that which would violateenvironmental regulations when the coal is burned. This, of course, willdepend to some extent on the equipment employed at the coal fired plantwhere the biotreated coal will be utilized. With respect to thebiooxidation of refractory sulfide ores or concentrates, the amount ofsulfide mineral that is permitted to remain in the ore will be dictatedby the amount that must be biooxidized to achieve economical recoveriesof the desired metal values from the ore or concentrate.

[0086] After the undesired compound has been reduced to a desiredconcentration, the bioreactor can be broken down and the biotreatedsolid material separated from the coarse substrates. After separation ofthe biotreated solid material, the coarse substrates can be reused.After one or more uses in the biotreatment process, a film of themicroorganism used in the biotreatment process will develop on thesubstrates. This biofilm will have the advantage of adaptation to anytoxic or inhibitory materials that are present in the solid materialbeing processed. It is therefore best to remove the biotreated solidmaterial in such a way as to not kill or entirely remove the biofilmthat has built up on the coarse substrates. The biofilm is also anefficient way to inoculate the next coating of solid material applied tothe coarse substrates. Finally, the adaptation of the microorganismafter having been through the process many times will also speed up therate at which the microorganism biodegrades or biooxidizes the undesiredcompound in the solid material being processed.

[0087] The present invention will now be described in further detail inconnection with a number of possible embodiments that can be employed inthe processing of refractory sulfide ores.

[0088] The second embodiment of the present invention is described inconnection with FIGS. 1 and 2. FIG. 1 illustrates a process flow chartfor liberating and recovering precious metal values from precious metalbearing refractory sulfide ores. For purposes of describing the processillustrated in FIG. 1, the sulfide mineral concentrate 22 used in thepresent embodiment is produced from a gold bearing refractory sulfideore. It follows, therefore, that the precious metal recovered in thepresent embodiment is gold. However, as one skilled in the art wouldunderstand, other precious metals, such as platinum and silver, can alsobe liberated and recovered from refractory sulfide ores using theprocess illustrated in FIG. 1. A combination of precious metals can alsobe recovered using the process according to the present embodiment ifthe refractory sulfide ore body used to produce the sulfide mineralconcentrate 22 contains more than one precious metal.

[0089] According to the process flow chart shown in FIG. 1, a pluralityof substrates 20 and a sulfide mineral concentrate 22 are added to arotating drum 24. Preferably the sulfide mineral concentrate 22 is in aslurry form and the plurality of substrates 20 are dry when added torotating drum 24 to improve the adhesion between the substrates 20 andthe concentrate 22. Optionally, a polymeric binding agent can be addedto rotating drum 24, although it is not necessary. As rotating drum 24rotates, the substrates 20 added to drum 24 are coated with the wetsulfide mineral concentrate 22 to form coated substrates 39. Coatedsubstrates 39 are then stacked to form static heap 26.

[0090] By using a slurry of concentrate in the coating process, the needand cost of drying the concentrate after its production is eliminated.Concentrate 22 and the plurality of substrates 20 can, however, be addedto rotating drum 24 in the dry state, in which case after the mixture isadded to drum 26 it is sprayed with water or an aqueous acid solution,preferably containing ferric ions, to cause the concentrate to stick tothe substrates. The benefit of using an aqueous acid solution containingferric ions to bind the concentrate to the surface of the substrates isthat it will begin to chemically oxidize the sulfide mineralconcentrate. Also it is acidic so that it will lower the pH of thecoated substrates 39 in preparation for biooxidation. The disadvantageof using such an acid solution is that it will increase the cost of theequipment used to form the coated substrates 39 because it must bedesigned to be acid resistant.

[0091] Sulfide mineral concentrate 22 is comprised of a plurality offine metal sulfide particles 40 which have finely disseminated gold andpossibly other precious metal values occluded within. Sulfide mineralconcentrate 22 will also typically contain fine particles of sand orother gangue material 42 from the refractory sulfide ore from whichconcentrate 22 is obtained. As a result, each of the coated substrates39 will be coated with the metal sulfide particles 40 and fines 42 asillustrated in FIG. 2.

[0092] The integrity of coated substrates 39 should be sufficient enoughto prevent a large number of blockages from forming in the flow channelswithin heap 26 while the metal sulfide particles 40 on the surface ofcoated substrates 39 are being biooxidized. Such blockages decreaseoxygen flow and bacteria migration within the heap and thereby reducethe rate of biooxidation.

[0093] Because metal sulfide particles 40 are hydrophobic, they willtend to stick to the dry substrates 20 without the use of a bindingagent such as a polymeric agglomeration aid. This assumes, however, thatthe metal sulfide particles 40 are of an appropriate size. Therefore, ifconcentrate 22 contains an adequate concentration of metal sulfideparticles 40, concentrate 22 will remain sufficiently adhered to coatedsubstrates 39, even without the use of a binding agent, to permit coatedsubstrates 39 to be handled while being stacked on heap 26 or placed intank 45, which is described later in connection with the embodimentillustrated in FIG. 5. Furthermore, coated substrates 39 should retaintheir integrity throughout the biooxidation process. When forming coatedsubstrates 39 without the use of a binding agent, therefore, it isimportant to use a sulfide mineral concentrate which has an adequateconcentration of metal sulfide particles and an appropriate particlesize.

[0094] While a polymeric binding agent can be used and would possiblyimprove the integrity of the coated substrates 39, the use of suchagents will increase the operating cost of the process.

[0095] Several factors need to be taken into consideration whendetermining the appropriate concentration of metal sulfide particles 40in concentrate 22. First, higher concentrations of metal sulfides aredesirable in the concentrate so that more metal sulfide particles 40 canbe processed per unit surface area of substrates 20. This isadvantageous in that as the loading of metal sulfide particlesincreases, the rate of biooxidation in heap 26 will tend to increase.Furthermore, because the precious metal values are occluded within themetal sulfide particles 40, higher concentrations of these particles inconcentrate 22 will tend to result in improved recovery rates for aparticular ore body, in addition to lowering the cost of processing theconcentrate per ounce of gold produced.

[0096] A second factor that weighs in favor of producing a concentrate22 that contains as much metal sulfides as practicable is that thepotential for the formation of blockages in the flow channels of heap 26is reduced by minimizing the amount of gangue material 42 in concentrate22. The reason being that the fine particles of gangue material 42 aremore hydrophilic than the fine metal sulfide particles 40, and, as aresult, they tend to adhere to the surface of substrate 20 lesstenaciously. The fine particles of gangue material 42 will, therefore,tend to migrate through the heap with the added bioleachant maintenancefluids during biooxidation, which in turn increases the likelihood thatblockages will form in flow channels of heap 26. Accordingly, as theconcentration of metal sulfide particles approaches 20 weight %, it maybe desirable or even necessary to use a polymeric agglomeration aid toensure sufficient integrity of the coated substrates 39 during handlingand biooxidation. On the other hand, by using a sulfide mineralconcentrate with at least about 40 weight % metal sulfide particles,coated substrates 39 can be readily formed without the use of apolymeric agglomeration aid and a high degree of loading of metalsulfide particles 40 per unit surface area is achieved.

[0097] At least two factors militate against using a sulfide mineralconcentrate 22 having a very high concentration of metal sulfideparticles 40. First, the cost of producing concentrate 22 is typicallyproportional to its concentration of metal sulfide particles. Thus, asthe concentration of metal sulfide particles 40 in concentrate 22increases, the cost of producing concentrate 22 will likewise increase.The added cost of producing very high grades of concentrate 22 may notbe offset by the incremental improvement in metal sulfides loading orintegrity of the coated substrates 39. Second, as the grade ofconcentrate increases, the amount of metal sulfide particles 40 thatremain with the tail fraction of the refractory sulfide ore willincrease. Because these metal sulfide particles contain occludedprecious metal values, any metal sulfide particles 40 that remain in theore tail will decrease the total recovery rate for the process.

[0098] Taking the above factors into consideration, sulfide mineralconcentrate 22 should contain at least 20 weight % metal sulfides toensure adequate handling characteristics and integrity duringbiooxidation. Preferably, however, the concentrate will contain at leastabout 40 weight % metal sulfides, and more preferably at least about 70weight %. Typically, concentrate 22 will contain between about 40 to 80weight % metal sulfides.

[0099] In general, as the particle size of the sulfide mineralconcentrate 20 decreases, the faster the biooxidation process willproceed. Smaller particle sizes also tend to result in improvedconcentrate grades. This is because it is typically easier to separatethe metal sulfide particles 40 from the bulk of gangue material as theparticle size of the ore is decreased. Sulfide mineral concentrate 22,therefore, preferably has a particle size of less than about 250 μm.Particles larger than 250 μm may not adhere to substrates 20 very wellwithout the use of a binding agent. In addition, unless the refractorysulfide ore from which concentrate 22 is produced is ground to at least100% passing 250 μm, it is difficult obtain a good separation of themetal sulfide particles 40 from the bulk of gangue material duringconcentration. This is especially true if flotation is used to formconcentrate 22, because particles larger than 250 μm do not float verywell. On the other hand, if the particle size of concentrate 22 is lessthan about 38 μm to 25 μm, the concentrate particles will tend to clumptogether during the coating process rather than form a relativelyuniform coating on coated substrate 39. These clumps of concentrate canblock air flow and bacteria migration during biooxidation, therebyreducing the rate of biooxidation in the heap.

[0100] Preferably the particle size of concentrate 22 is about 100%passing 106 μm to 75 μm. Particles in this size range adhere well tosubstrates 20, and the incremental improvements which can be achieved inthe rate of biooxidation and the concentrate grade with finer particlesizes are rarely justified by the added grinding costs of producingthem.

[0101] Sulfide mineral concentrate 22 can be produced from any preciousmetal bearing refractory sulfide ore body being mined using techniqueswell known in the art and thus need not be explained in detail here. Theproduction of concentrate 22, however, will typically include thecrushing and grinding of the refractory sulfide ore to an appropriateparticle size followed by one or more gravity separations or one or moresulfide flotations.

[0102] Some potential refractory sulfide ore bodies may already be ofsufficient grade such that further concentration is not required. Suchore bodies may include tailings or waste heaps at existing mines. Whenthese types of ores are processed, the sulfide mineral concentrate needonly be transported to the location of the biooxidation facility andpossibly some additional comminution to achieve the desired particlesize.

[0103] With respect to gold concentration, the process according to thepresent embodiment can be performed economically even if concentrate 22contains as little as 5 g Au/metric ton of concentrate (or an equivalenteconomic value of other precious metal values). This number of coursewill vary to a large extent based on the cost of producing concentrate22 and the prevailing price of gold. As those skilled in the art willrecognize, however, traditional autoclaves or stirred tank bioreactorscannot come close to economically processing a sulfide mineralconcentrate having such a low concentration of gold.

[0104] Many different materials can be used for substrates 20. Preferredsubstrates include coarse refractory sulfide ore particles; lava rock,gravel, and rock which includes a mineral carbonate component. Thepurpose of the substrates 20 is to provide a support with a relativelylarge surface area upon which the concentrate 22 can reside during thebiooxidation process. The surface area of each substrate 20 in effectacts as a small surface bioreactor during biooxidation. Therefore, whena large number of coated substrates 39 are stacked in heap 26 forbiooxidation, a nonstirred surface bioreactor is created that has a verylarge total surface area.

[0105] The total surface area of the bioreactor or heap 26 can beincreased by decreasing the particle size of substrates 20, usingsubstrates that have a rough, nonuniform surface morphology and/orincreasing the number of coated substrates 39 stacked on heap 26. Theadvantage of increasing the total surface area of the substrates 30within heap 26 is that the amount of concentrate 22 that can be loadedon substrates 30 increases proportionately, which in turn increases theamount of concentrate 22 that can be biooxidized in a particular heap26.

[0106] The preferred particle size range for substrates 20 is nominallyfrom about +0.62 cm to about −2.5 cm with particles less than about 0.3cm removed by screening or other suitable method. However, substrates 30having a particle size down to approximately +600 μm can be used. Whileincreased loading is achieved with smaller substrate particle sizes,increased air flow, fluid flow and heat dissipation is achieved withlarger particle sizes. The nominal +0.62 to −2.5 cm size range providesa good compromise between concentrate loading and ensuring adequate airflow, fluid flow, and heat dissipation.

[0107] Substrates 20 are preferably loaded with as much concentrate 22during the coating process as possible to maximize the processthroughput. The amount of concentrate 22 that can be loaded onsubstrates 20 will depend on particle size and surface morphology of thesubstrates 20. Coarse substrates 20 and sulfide mineral concentrate 22should, therefore, be added to rotating drum 24 in sufficient quantitiesto maximize the amount of sulfide mineral concentrate 22 loaded on eachsubstrate 39 while minimizing the formation of agglomerates of thesulfide mineral concentrate particles. Clumps or agglomerates of thesulfide mineral concentrate 22 particles may be formed if the particlesize of the concentrate is too fine, as discussed above, or if an excessamount of the concentrate is added to drum 24. To ensure adequateloading of substrates 20 while simultaneously avoiding formation ofagglomerates of the concentrate particles, preferably approximately 10to 30 weight % concentrate is added to rotating drum 24, which willresult in a loading of approximately 10 to 30 weight % of concentrate 22on coated substrates 39.

[0108] In forming coated substrates 39, it is desirable to maintain themoisture content of concentrate 22 within the range of 5 to 30 weight %.If the moisture content of the concentrate is below 5 weight %, theconcentrate will not adhere properly to the substrates, and if themoisture content exceeds 4.0 weight %, the concentrate slurry will betoo thin to form a thick enough coating on the substrate. This wouldlimit the amount of concentrate that would adhere to the substrates 20.

[0109] Although other means of heap construction may be used, conveyorstacking is preferred. Conveyor stacking minimizes compaction of thecoated substrates within the heap. Other means of stacking such as enddumping with a dozer or top dumping can lead to regions of reduced fluidflow within the heap due to increased compaction and degradation of thecoated substrates.

[0110] If desired, heap 26 can be provided with perforated pipes 27connected to an air supply source (not shown) in order to increase theair flow within the heap. Increasing the air flow within heap 26 willincrease the rate of biooxidation and improve the rate at which heat isdissipated from the heap. Furthermore, because of the large air andfluid flow channels between the coated substrates 39, the air supplysource connected to perforated pipes 27 can be a low cost blower ratherthan a more expensive compressor.

[0111] Heap 26 is preferably inoculated with a bacteria capable ofbiooxidizing metal sulfide particles 40 while coated substrates 39 arebeing stacked on to heap 26 or immediately after formation of heap 26 orafter the pH of heap 26 has been lowered to below 2.5. The followingbacteria may be used in the practice of the present invention:

[0112]Thiobacillus ferrooxidans;Thiobacillus thiooxidans; Thiobacillusorganoparus; Thiobacillus acidophilus; Leptospirillum ferrooxidans;Sulfobacillus thermosulfidooxidans; Sulfolobus acidocaldarius;Sulfolobus BC; Sulfolobus solfataricus and Acidianus brierleyi and thelike.

[0113] These bacteria are all available from the American Type CultureCollection or like culture collections. Whether one or more of the abovebacteria and the particular bacteria selected for use in the presentprocess will depend on factors such as the type of ore being processedand the expected temperatures in heap 26 during biooxidation. Theseselection criteria, however, are well within the skill of those in theart and need not be described in detail here. The most common andpreferred bacteria for biooxidation is Thiobacillus ferrooxidans.

[0114] During the biooxidation of the metal sulfide particles 40 coatedon the surface of the coated substrates 39, additional inoculant andmicrobial nutrient solutions can be supplied through a sprinkler system28. Additions of these bioleachant maintenance solutions will typicallybe made in response to certain performance indicators used to monitorthe progress of the biooxidation process.

[0115] The rate of biooxidation is preferably monitored throughout thebiooxidation process based on selected performance indicators such asthe solubilization rate of arsenic, iron, or sulfur, or the oxidationrate of sulfides which can be calculated therefrom. Other biooxidationperformance indicators that may be used include measuring pH, titratableacidity, and solution Eh. Preferably the bioleachate off solution thatpercolates through the heap is collected at drain 29 and recycled to thetop of heap 26. This minimizes the amount of fresh water required by thebiooxidation process. And, because the bioleachate off solution will beacidic and contain a high concentration of ferric ions, itsreapplication to the top of heap 26 is advantageous to the biooxidationprocess. However, the effluent solution generated early in thebiooxidation process will also contain significant concentrations ofbase and heavy metals, including components that lead to microbialinhibition. As the inhibitory materials build up in the bioleachate offsolution, the biooxidation process is retarded. Indeed, continuedrecycling of an off solution without treatment can lead to a build-up ofinhibitory materials sufficient to stop the biooxidation processaltogether.

[0116] To minimize the build-up of inhibitory materials and thus theireffect on the biooxidation process, the off solution can be treated inacid circuit 30 prior to recycling to remove the inhibitory materialswhen their concentration becomes excessive. One method of conditioningthe bioleachate off solution before recycling comprises raising its pHabove 5, removing any precipitate that forms and then lowering its pH toa pH appropriate for biooxidation using an untreated portion of the offsolution or other acid solution. Such a conditioning process isdisclosed in U.S. pat. application Ser. No. 08/547,894, filed Oct. 25,1995 by Kohr et al, which is hereby incorporated by reference.

[0117] The bioleachate off solution will tend to be very acidic in thepresent invention. This is because a concentrate having a relativelyhigh concentration of metal sulfide minerals is being biooxidized ratherthan an entire ore. As a result, the biooxidation process according tothe present invention will tend to produce large amounts of excess acid.That is the process will produce more acid than can be practicallyrecycled to the top of heap 26. This excess acid must be disposed of orused for other purposes. One possible use for the excess acid is in acopper oxide ore leaching process because sulfuric acid is an effectivelixiviant for copper oxide ores. However, the sulfuric acid solutionproduced as a byproduct of the present process will also typicallycontain a high concentration of ferric ions. This also makes it aneffective lixiviant for some copper sulfide ores such as chalcocite. Theferric ion in the acid solution chemically oxidizes the copper sulfideminerals to cause their dissolution. Thus, the excess acid from thepresent process can be beneficially used in a copper leaching operationto avoid the neutralization costs associated with disposal whilesimultaneously reducing the acid costs for the copper leachingoperation.

[0118] After the biooxidation reaction has reached an economicallydefined end point, that is after the metal sulfide particles 40 on thesurface of the coarse substrates 20 are biooxidized to a desired degree,the heap is broken down and the biooxidized concentrate 22 is separatedfrom the coarse substrates 20. Prior to breaking the heap down, however,the heap will typically be drained and then washed by repeated flushingswith water. The number of wash cycles employed is typically determinedby a suitable marker element such as iron and the pH of the washeffluent.

[0119] Separation can be accomplished by placing the coated substrates39 on a screen and then spraying the coated substrates with water.Alternatively, the coated substrates can be tumbled in water using atrommel.

[0120] Following separation, gold is extracted from the biooxidizedconcentrate 22. This can be accomplished using a number of techniqueswell known in the art. Typically, however, the biooxidized concentratewill be leached with a lixiviant such as cyanide in a carbon-in-pulp ora carbon-in-leach process. In these processes, the lixiviant dissolvesthe liberated gold or other precious metal values that are then adsorbedonto activated carbon as is well known in the art.

[0121] If cyanide is used as the lixiviant, the concentrate will need tobe neutralized prior to leaching. To avoid the need for neutralization,thiourea can be used as the lixiviant to extract the gold from thebiooxidized concentrate. The thiourea extraction process can be improvedby adjusting the Eh of the leach solution using sodium metabisulfite asdisclosed in U.S. Pat. No. 4,561,947, which is incorporated herein byreference. If thiourea is used as the lixiviant, preferably a syntheticresin, rather than activated carbon, is used to adsorb the dissolvedprecious metal values from the lixiviant solution.

[0122] After the liberated gold or other precious metal values areextracted from the biooxidized concentrate, the biooxidized concentrateis taken to a waste or tailings pile 36 and gold is recovered from thecarbon or synthetic resin using techniques well known in the art.

[0123] The coarse substrates 20 which have been separated from thebiooxidized concentrates can be recycled to the rotating drum for a newcoating of sulfide mineral concentrate 22. Substrates 20 can be reusedso long as they retain their mechanical integrity. If coarse refractorysulfide ore particles are used for substrates 20, they are preferablyprocessed at some point, preferably after one to three cycles, torecover liberated gold values.

[0124] As illustrated in FIG. 2, coarse refractory sulfide oresubstrates 20 will contain metal sulfide particles 40 which containoccluded gold and other precious metal values. After one to three cyclesthrough the process, many of the metal sulfide particles 40 within thecoarse ore substrates 20 will be partially biooxidized. Rather thancontinuing to recycle the coarse ore substrates in this situation andallow the liberated gold values to go unclaimed, the coarse oresubstrates can be processed to recover their gold values. This ispreferably accomplished by grinding the coarse ore substrates ingrinding circuit 32 to a particle size suitable to permit the metalsulfide particles to be separated from the bulk of the gangue material.A concentrate 22 of the metal sulfide particles 40 from the groundcoarse ore substrates is then produced in the sulfide concentrator 34.Preferably sulfide concentrator 34 is a flotation cell and thebiooxidized coarse ore substrates are ground to a size appropriate forsulfide flotation and coating on substrates 20. The concentrate 22produced from the ground ore substrates is then combined with the supplyof sulfide mineral concentrate 22 from which it is coated on a secondplurality of coarse substrates 20 and added to a new heap 26 for furtherbiooxidation.

[0125] The flotation tail from sulfide concentrator 34 should be treatedin the gold extraction process along with the biooxidized concentrate 22from heap 26. The flotation tail will contain a number of fully andpartially oxidized metal sulfide particles that did not float. Theseoxidized particles will contain significant gold values, and as much ofthese gold values will already be liberated, they can be readily leachedfrom the flotation tail using cyanide or thiourea. After lixiviation,the flotation tail is disposed of along with the biooxidized concentratewhich has gone through gold extraction in waste or tailings pile 36.

[0126] Refractory sulfide coarse ore substrates 20 that have gonethrough the biooxidation process can alternatively be processed simplyby grinding followed by lixiviation. This process alternative, however,will result in a lower overall recovery, because many of the metalsulfide particles 40 within the coarse ore substrates will not besufficiently oxidized to liberate their entrapped gold values.

[0127] With respect to material selection for substrates 20, there areseveral advantages of using coarse refractory sulfide ore particles.

[0128] First, the refractory sulfide ore body being mined will typicallyhave to go through several crushing and grinding steps before anappropriate particle size is achieved for producing concentrate 22. As aresult, coarse refractory sulfide ore substrates can be removed from anappropriate stage of the crushing process, which makes coarse refractorysulfide ore particles an inexpensive source of substrates 20.

[0129] Second, as illustrated in FIG. 2 and discussed above, if coarserefractory sulfide ore is used as the substrate material, it willcontain metal sulfide particles 40. These metal sulfide particles willbe partially biooxidized during the biooxidation process, and, if thecoarse ore particles are recycled through the process several times, themetal sulfide particles 40 will eventually become sufficientlybiooxidized to permit recovery of their precious metal values.

[0130] A third advantage, which is somewhat related to the second, isthat a fraction of the iron sulfide or other metal sulfide particles 40in the refractory sulfide ore are so fine that they will not float verywell in the concentration process. By using coarse particles of the orefor substrates 20, these very fine metal sulfide particles will bechemically oxidized over time by the ferric ion in the bioleachant.Then, when the coarse ore particles are eventually ground and floated toproduce a concentrate of metal sulfide particles, the oxidized finemetal sulfide particles will end up in the flotation tails. Because theflotation tails are leached with cyanide or other lixiviant, theliberated gold values from these very fine sulfide particles will berecovered. On the other hand, if the coarse ore particles were not usedas substrates 20 prior to grinding and flotation, the very fine metalsulfide particles would still end up in the flotation tails whenproducing concentrate 22. However, because these very fine sulfideparticles would not be partially biooxidized at this point, theiroccluded gold values cannot be recovered by lixiviation.

[0131] A fourth advantage of using refractory sulfide coarse ore assubstrates 20 is that the metal sulfide particles in the biooxidizedsupport material will be easier to float following biooxidation. This isbecause the surface of the metal sulfide particles is altered during thebiooxidation process. Thus, after the coarse ore support material hasbeen reused several times and it is ground and floated to produce asulfide mineral concentrate, improved flotation results can be achieved.

[0132] If the coarse ore particles also contain a carbonate mineralcomponent, a fifth advantage exists for using coarse refractory sulfideore particles as the coarse substrates 20. Carbonate minerals tend to bevery acid consuming. As a result, ores which contain these minerals havetraditionally required a lot of acid conditioning prior to biooxidation.Acid conditioning of these ores is required to remove or reduce thecarbonate mineral component prior to biooxidation so that thebiooxidation reaction can proceed. And, while coarse refractory sulfideore particles in general tend to biooxidize very slowly—often taking upto nine months or more—if lots of carbonate minerals are included in theore, without preconditioning, the coarse ore particles may neverbiooxidize. In the process according to the present invention, however,coarse refractory sulfide ore particles that contain carbonate mineralscan be advantageously used for substrates 30. During the biooxidationprocess, the acid produced from the biooxidation of the concentrate 22on the surface of the coarse ore substrates will slowly neutralize thecarbonate minerals in the substrates. A byproduct of the neutralizationprocess is carbon dioxide, which the autotrophic bacteria used in thepresent invention can use as a source of carbon to carry out metabolicsynthesis. The carbon dioxide production, therefore, will promotebacteria growth in heap 26, which in turn increases the rate ofbiooxidation of concentrate 22. Thus, by using coarse ore that containscarbonate minerals for support material 20, the coarse ore will beslowly neutralized for future biooxidation and bacteria growth in heap26 will be promoted. A concomitant benefit, as noted above, will be thebiooxidation of the very fine nonfloatable sulfide particles that are inthe coarse ore.

[0133] As those skilled in the art will recognize, the coarse refractorysulfide ore particles used for substrates 20 do not have to originatefrom the same ore body as that used to produce concentrate 22. In fact,in some situations, it may be beneficial to use a concentrate 22 fromone ore body and coarse ore substrates 20 from another. For example, oneore body may be easily concentrated or already have the characteristicsdesirable of a concentrate and another ore body may have a highconcentration of carbonate minerals. In such a situation, it would beadvantageous to use the first ore body to produce concentrate 22 and thesecond ore body to produce substrates 20. In this way, the ore from thesecond ore body can be neutralized in preparation for biooxidation whilesimultaneously improving the biooxidation results of the concentratefrom the first ore body. Similarly, if an ore body contains a highconcentration of metal sulfides that are difficult to float, improvedflotation results can be achieved by first using the ore as coarse oresubstrates 40 in the process according to the present invention.

[0134] Other preferred materials for substrates 20 include lava rock,gravel, and coarse rock containing carbonate minerals. These types ofsubstrates will typically be used when the refractory sulfide ore bodybeing mined is a waste heap or tailings pile, and, as a result, the orehas already gone through crushing and grinding.

[0135] An advantage of using lava rock is that it has a very rough,nonuniform surface morphology which increases the overall surface areaof the substrates 30 for a particular particle size. Thus, for a givenparticle size, lava rock can be loaded with more concentrate than othersubstrates having a smoother surface.

[0136] Gravel, while typically having a fairly smooth surface, is aninexpensive substrate material. Coarse rock containing carbonateminerals is advantageous, because it will slowly release carbon dioxideas the acid from the biooxidation process neutralizes the carbonateminerals as explained above. This type of substrate would preferably bereused in the process only as long as it continues to release carbondioxide during the biooxidation process.

[0137] A third embodiment of the present invention is now described inconnection with FIG. 3. The process according to the present embodimentis essentially a variation on the embodiment described in connectionwith FIG. 1. Accordingly, like items are referred to with the samereference numbers, and the description and considerations expressed withrespect to these items in connection with FIG. 1 will be understood toapply equally to the present embodiment.

[0138] As with the second embodiment, the process according to thepresent embodiment can be used to liberate and recover precious metalvalues from a precious metal bearing refractory sulfide ore. Forpurposes of the present description, however, it is assumed that thesulfide mineral concentrate 22 is produced from a gold bearingrefractory sulfide ore.

[0139] According to the present embodiment, a plurality of substrates 20are coated with a sulfide mineral concentrate 22 in rotating drum 24 toproduce a plurality of coated substrates 39. The plurality of coatedsubstrates 39 are then stacked to form heap 26, which is used as a largenonstirred surface bioreactor.

[0140] The various considerations discussed above in connection withsubstrates 20, sulfide mineral concentrates 22, the formation of coatedsubstrates 39, and the formation of heap 26 are all equally applicablehere.

[0141] After heap 26 is formed the heap is inoculated with abiooxidizing bacteria to initiate the biooxidation process. As thebiooxidation process proceeds, additional sulfide mineral concentrate 22can be added to the top of heap 26. An advantage of adding additionalsulfide mineral concentrate 22 to the top of heap 26 throughout thebiooxidation process is that the amount of concentrate processed in theheap can be increased before tearing down and rebuilding. Furthermore,if coarse refractory sulfide ore is used for substrates 20, concentrate22 will tend to biooxidize more quickly than the metal sulfide particles40 found in the coarse ore. Thus, by adding additional concentrate 22 tothe top of heap 26, the degree of biooxidation of the coarse oresubstrates can be increased before heap tear down. In addition, byadding the sulfide mineral concentrate 22 to the top of heap 26, acidand ferric ions produced during its biooxidation will migrate to thelower part of the heap where bacterial growth may be inhibited due totoxins, which have not been washed from the ore early in thebiooxidation process, or due to the lack of oxygen. As a result,biooxidation of the sulfide mineral concentrate and coarse oresubstrates will proceed even if bacterial growth is not favored in thisregion.

[0142] There is another advantage to adding sulfide mineral concentrate22 to the top of heap 26 after it has been undergoing biooxidation forsome time, because such additions will increase the biooxidation rate inthe heap. In the later stages of biooxidation of the coated substrates39, most of the exposed and reactive sulfides will have already beenoxidized, resulting in a slow down in the rate of biooxidation. Thisslow down in the rate of biooxidation can lead to a drop in iron levelsand an increase in pH within heap 26. Addition of fresh reactive sulfidemineral concentrate 22 to the top of heap 26 can restart an activebiooxidation process due to the high ferric levels produced from thebiooxidation of the added concentrate, which in turn will increaseindirect chemical leaching of the sulfide mineral concentrate 22 coatedon substrates 20 and of metal sulfide particles imbedded in coarse oresubstrates 20.

[0143] Fresh concentrate 22 can be added to the top of heap 26 until theflow channels within the heap begin to become plugged with theconcentrate and biooxidized residue from the concentrate.

[0144] A second variation in the present embodiment from that in FIG. 1is with respect to how the precious metal values are recovered from theheap following biooxidation. In the present embodiment, instead oftearing down the heap and then separating the biooxidized concentratefrom the heap for gold extraction, gold is extracted from thebiooxidized concentrate—and if a coarse ore substrate is used, from thesubstrates—by directly lixiviating the heap with a precious metallixiviant. Preferably the lixiviant is one that functions at a low pH,such as thiourea, so the heap does not need to be neutralized prior tolixiviation. Furthermore, by using thiourea or other acid compatiblelixiviant, the liberated gold values can be extracted from the heap onan intermittent basis. For example, heap 26 can be biooxidized for aperiod, liberated gold values extracted with an appropriate lixiviant,and then the biooxidation process resumed. A fresh concentrate 22 ispreferably added to the top of heap 26 in slurry form with theresumption of the biooxidation process.

[0145] Gold is extracted from heap 26 by first allowing the bioleachatesolution to drain from the heap to acid circuit 30 following a desireddegree of biooxidation. After the heap is drained, an acid compatiblelixiviant such as thiourea is pumped from the lixiviant supply 38 to thesprinkler system 28 where it is dispersed onto heap 26. As the lixiviantpercolates through the heap, it dissolves liberated gold values from thesulfide mineral concentrate 22 and coarse ore substrates. The loadedlixiviant then collects at drain 29 where it diverted from the acidcircuit to a gold removal process 44, which preferably comprisesadsorbing the dissolved gold onto activated carbon or a synthetic resin.The barren lixiviant is then recycled to the lixiviant supply 38 andgold is recovered from the loaded activated carbon or synthetic resin.Processes for stripping adsorbed gold values from activated carbon andsynthetic resin are well known in the art and need not be describedherein.

[0146] A process according to a fourth embodiment of the presentinvention is illustrated in FIG. 4.

[0147]FIG. 4 illustrates a process for liberating and recovering metalvalues from a sulfide ore. As the process according to the presentembodiment has certain similarities to the embodiment described inconnection with FIG. 1, like items have been referred to with the samereference numbers. Furthermore, the description and considerationsexpressed with respect to these items in connection with FIG. 1 will beunderstood to apply equally to the present embodiment.

[0148] According to the present embodiment, a sulfide mineralconcentrate 22 is first produced from a sulfide ore. Concentrate 22 iscomprised of a plurality of fine metal sulfide particles 40 and fineparticles of sand or other gangue material 42.

[0149] Many different sulfide ores can be used to produce sulfidemineral concentrate 22. Foremost amongst the sulfide ores that can betreated in the present process are sulfide ores that contain sulfideminerals of base metals such as copper, zinc, nickel, iron, molybdenum,cobalt, or uranium. The metal values of interest in these ores arepresent in the metal moiety of the sulfide mineral particles in the ore.The metal values which are liberated and recovered, therefore, willdepend on the specific sulfide minerals present in concentrate 22produced from the ore. For example, if the sulfide ore used to produceconcentrate 22 contains chalcocite, bornite, and/or chalcopyrite, thenthe metal values recovered will be that of copper. On the other hand, ifconcentrate 22 is a concentrate of sphalorite, the metal valuesrecovered will be that of zinc.

[0150] After concentrate 22 is produced, sulfide mineral concentrate 22is then coated on a plurality of substrates 20 to form coated substrates39. This is accomplished as described in connection with FIG. 1 byadding a plurality of dry substrates 20 and a slurry of concentrate 22to rotating drum 24, or, alternatively, by adding a plurality of drysubstrates 20 and concentrate 22 to rotating drum 24 and then sprayingthe mixture with an aqueous solution. The plurality of coated substrates39 produced in rotating drum 24 are stacked to form heap 26, which formsa large nonstirred surface bioreactor.

[0151] The various considerations discussed above in connection withsubstrates 20, sulfide mineral concentrates 22, the formation of coatedsubstrates 39, and the formation of heap 26 are all equally applicablehere.

[0152] After heap 26 is formed, the heap is inoculated with abiooxidizing bacteria to initiate the biooxidation process. As the metalsulfide particles 40 in concentrate 22 biooxidize, the metal moiety ofthe sulfide particles dissolves in the bioleachate solution as itpercolates through the heap. After the bioleachate solution percolatesthrough the heap, it is collected at drain 29. The bioleachate solutionis then processed to recover one or more desired base metal values byremoving them from the bioleachate solution using techniques well knownin the art.

[0153] Following recovery of the desired metal values from thebioleachate solution, the solution can be processed in acid circuit 30to remove any excess toxins as described in connection with FIG. 1 andthen reapplied to the top of heap 26.

[0154] Once the biooxidation reaction has reached an economicallydefined end point, that is after the metal sulfide particles 40 on thesurface of the coarse substrates 20 are biooxidized to a desired degree,the heap is broken down and the biooxidized concentrate separated fromthe coarse substrates 20. The biooxidized concentrate is then disposedof in waste or tailings pile 36. It is to be understood, however, thatwhile the present embodiment has been described in terms of liberatingand recovering base metal values from the metal moiety of the metalsulfide particles 40 in sulfide mineral concentrate 22, sulfideparticles 40 can also include occluded precious metal values. Afterbiooxidation of concentrate 22, therefore, any precious metal valuesthat are liberated in concentrate 22 can be extracted and recovered asdescribed in connection with FIG. 1 prior to the disposal of thebiooxidized concentrate.

[0155] The coarse substrates 20 which have been separated from thebiooxidized concentrate can be recycled to the rotating drum for a newcoating of sulfide mineral concentrate 22. Alternatively, if coarsesulfide ore particles are used for substrates 20, they are preferablyprocessed after one or more cycles through the process to form a sulfidemineral concentrate of any metal sulfide particles 40 which remainunoxidized in the coarse ore substrates. Sulfide mineral concentrate 22is produced from the biooxidized coarse ore substrates as described inconnection with the second embodiment.

[0156] A process according to a fifth embodiment of the presentinvention is illustrated in FIG. 5. The process illustrated in FIG. 5 isfor liberating and recovering precious metal values from precious metalbearing refractory sulfide ores using a nonstirred bioreactor. Theprocess comprises producing a concentrate 22 of metal sulfide particles40 from the refractory sulfide ore being processed. Concentrate 22 isthen coated on a plurality of coarse substrates 20 to form coatedsubstrates 39 using rotating drum 24 as described in connection with thesecond embodiment. After formation, coated substrates 39 are placed in atank 45 for biooxidation. By biooxidizing substrates 39 in tank 45, alarge nonstirred surface bioreactor is created which has a very largesurface area. Thus, tank 45 takes the place of heap 26 in the processaccording to the second embodiment. Accordingly, the variousconsiderations discussed above in the second embodiment with respect tosubstrates 20, sulfide mineral concentrates 22, the formation of coatedsubstrates 39, and the formation of heap 26 are all equally applicableto the biooxidation of coated substrates 39 in tank 45 in the presentembodiment.

[0157] During the biooxidation of concentrate 22 on coated substrates39, bioleachant maintenance solutions are added to the tank from the topusing any of a number of well known techniques. The bioleachate solutionthat percolates through the tank is drained from the tank and processedin acid circuit 30 as described in connection with FIG. 1 prior to reusein the process.

[0158] Air can be blown into the tank during the biooxidation process toimprove the oxygen levels in the bioreactor and to improve heatdissipation. Air is preferably blown into tank 45 through a series ofperforated pipes 46 which are connected to a blower (not shown).

[0159] If desired, additional concentrate 22 can be added to the top ofthe coated substrates 39 in tank 45 throughout the biooxidation process.As described above in connection with the third embodiment, by addingadditional concentrate to the bioreactor during the biooxidationprocess, the rate of biooxidation within the bioreactor can bemaintained at a high level throughout the biooxidation process.

[0160] An advantage of using tank 45 over heap 26 for the bioreactor isthat it makes separation of the biooxidized concentrate 22 from thesubstrates 20 easier. After the concentrate 22 is biooxidized to adesired end point, separation of the biooxidized concentrate from thesubstrates is accomplished by filling the tank with water, and thenrapidly draining the tank. The biooxidized concentrate will be carriedwith the draining water. This process can be repeated several times toimprove separation results. Tank 45 is also preferably equipped with ascreen in the bottom of the tank which has a mesh size that is less thanthe size of the substrates, but larger than the concentrate particlesize to aid the separation process.

[0161] After separation, the biooxidized concentrate is leached with aprecious metal lixiviant to extract the liberated gold or other preciousmetal values. The dissolved gold values are then recovered from thelixiviant by contacting the solution with activated carbon or asynthetic resin. Preferably the lixiviation is carried out in thepresence of the activated carbon or a synthetic resin so that thedissolved gold values are immediately removed from the solution as theyare dissolved. The gold adsorbed on the activated carbon or syntheticresins can be recovered using techniques well known in the art.

[0162] Once the precious metal values have been extracted from thebiooxidized concentrate, the concentrate can be disposed of in waste ortailings pile 36.

[0163] As in the second embodiment, the coarse substrates 20 that havebeen separated from the biooxidized concentrates can be recycled to therotating drum for a new coating of sulfide mineral concentrate 22.Substrates 20 can be reused as long as they retain their mechanicalintegrity. If coarse refractory sulfide ore particles are used assubstrates 20, they are preferably processed at some point, preferablyafter one to three cycles, to recover liberated gold values. This isaccomplished in the same manner as described in connection with thesecond embodiment.

[0164] The preferred embodiments of the invention having been described,various aspects of the invention are further amplified in the examplesthat follow. Such amplifications are intended to illustrate theinvention disclosed herein, and not to limit the invention to theexamples set forth.

EXAMPLE 1

[0165] A sample of low grade (3.4 ppm) gold ore, which was known to berefractory to leaching with cyanide due to sulfides, was crushed. Theore was then separated into a −0.62 cm fraction (47.4 wt %) and a −0.31cm fraction (remainder). The −0.31 cm fraction was then further groundto 95% passing a 75 μm sieve to aid in producing a refractory pyriteconcentrate by flotation.

[0166] Water was added to the ground sample until it reached a 30% pulpdensity. The ore pulp was then adjusted to a pH of 10 and treated withNa₂SiO₃ at 6 Kg/tonne of ore for 12 hours to remove the clay material.The clay material was removed as the fraction that did not settle after12 hours.

[0167] Because clays can cause problems with flotation, a step thatpermits the non clay material to settle out was added to remove the clayfraction before floating the sample.

[0168] The clay fraction was under 3% of the total ore weight, yet itcontained almost 5% of the gold in the ore. The removal and subsequentflotation of the clay fraction produced a very small weight fraction(0.1% of the total ore weight), but it contained over 17 ppm gold.Cyanide leaching of the clay flotation tail extracted over 76% of thegold contained therein. The total amount of gold contained in the clayflotation tail was 1.08 ppm.

[0169] Before floating, the main fraction of ground ore (+5 μm to −75μm) was conditioned with CaSO₄ at 2.0 Kg/tonne for ten minutes by mixingin a Wemco flotation cell. This was followed by 10 minutes of mixingwith Xanthate at 100 g/tonne which was then followed by 5 minutes ofmixing with Dowforth D-200 at 50 g/tonne. The sample was then floatedfor 20 minutes at a pulp density of 30%. Four Kg of the main fractionwas processed in 8 separate batches of 500 g each. The sulfideconcentrates obtained from these flotations were collected and combinedand refloated in a column.

[0170] Three fractions were collected, the tail from the Wemco float,the tail from the column float, and the sulfide concentrate, each ofthese fractions were dried and weighed. The tail from the Wemco floatwas 35.4 wt % of total ore weight and contained 1.88 ppm of gold.Cyanide leaching of this fraction yielded 67% of its gold. This washigher than the recovery for cyanide leaching of the whole ore, whichwas 63%. The column tail contained 3.56 ppm of gold. The gold recoveryfrom this fraction by cyanide leaching was 76.6%.

[0171] The sulfide concentrate weighed 753 g which represented 8.8% ofthe total ore (+0.31 cm and −0.31 cm fractions). Analysis of a smallfraction of the concentrate indicated it contained 6.5 ppm of gold. Thisfraction was coated on to the 47.4 weight percent of the +0.31 cm ore.The dry pyrite concentrate was spread over the surface of the coarse oreby rolling in a drum rotating at 30 rpm while spraying a mixture of2,000 ppm ferric ion and 1% Nalco #7534, which is an agglomeration aid.The pH of the solution was 1.8.

[0172] The mixture of concentrate on coarse ore support was placed intoa 3 inch column. Air and liquid were introduced from the top. The columnwas inoculated with 10 ml of Thiobacillus ferrooxidans bacteria at anO.D. of 2.6 or about 1.1×10¹⁰ bacteria per ml.

[0173] The bacteria were grown in an acidic nutrient solution containing5 g/l ammonium sulfate and 0.83 g/l magnesium sulfate heptahydrate. ThepH of the solution was maintained in the range of 1.7 to 1.9 byadjustment with sulfuric acid (H₂SO₄). The solution also contained ironat 20 g/liter in the form of ferric and ferrous sulfate.

[0174] The bacteria were added to the top of the column after the pH wasadjusted to a pH of 1.8. The liquid, introduced to the top of the columnthroughout the experiment, was pH 1.8; with 0.2×9 K salts and 2,000 ppmferric. The extent of iron oxidation was determined by analysis of thesolution eluting off the column minus the iron introduced by the 2,000ppm ferric feed.

[0175] The composition of the standard 9 K salts medium for T.ferrooxidans is listed below. The concentrations are provided ingrams/liter. (NH₄)SO₄ 5 KCl 0.17 K₂HPO₄ 0.083 MgSO₄.7H₂O 0.833Ca(NO₃).4H₂O 0.024

[0176] The notation 0.2×9 K salts indicates that the 9 K salt solutionstrength was at twenty percent that of the standard 9 K salt medium.

[0177] After 60 days the amount of iron leached off of the columnindicated that about 50% of the pyrite had been biooxidized. Theexperiment was stopped and the mixture separated into a +600 μm fractionand a −600 μm fraction. Each fraction was ground to 95% minus 75 μm andthen leached with a 500 ppm cyanide solution in a 96-hour bottle rollanalysis. Activated carbon was added to the bottle roll test to absorbany dissolved gold.

[0178] The gold recovery of the −30 mesh fraction was 83.7%. The −30mesh material had an increased head gold value of 8.87 ppm due to lossof pyrite weight. The coarse +30 mesh fraction, on the other hand, had agold recovery of 57% and a head gold value of 2.24 ppm. This indicatedthat the concentrate pyrite that was coated on the outside of the coarserock had biooxidized faster than the coarse fraction of the rock.

EXAMPLE 2

[0179] Another comparative test was made. In this example, thebiooxidation rates of ore size fractions were compared. The ore, whichwas provided by the Ramrod Gold Corporation, was crushed to 1.9 cm. The−0.31 cm ore fraction was removed and used to form a concentrate. Theore sample had less than 0.08 oz. of gold per ton of ore (2.7 ppm). Thesample contained both arsenopyrite and pyrite. The concentrate was madeby ball milling 5 Kg of the −0.31 cm inch ore until it passed −75 μm,the ball milled ore was then floated with Xanthate to form a pyriteconcentrate. Before flotation clay was removed by settling with Na₂SiO₃at 6 Kg/tonne of ore for 8 hours or more. The flotation was done insmall batches of 500 g each in a laboratory Wemco flotation cell.Potassium Amyl Xanthate was used as a collector at a concentration of100 g/tonne along with sodium sulfide at 1.5 Kg/tonne and Dowfroth D-200at 50 g/tonne. The pyrite concentrate constituted 4.5% of the weight ofthe −0.31 cm ore fraction. However, this ore fraction contained over 80%of the gold and pyrite for the milled ore. The concentrate containedapproximately 17.4% iron, 15.7% sulfur and approximately 40 ppm gold.The +0.31 cm ore contained 0.9% iron and 0.18% sulfur.

[0180] A sample of 140 g of this concentrate was coated onto 560 g of+0.31 cm coarse ore. The concentrate was added as a dry powder to thecoarse ore. The mixture was then rotated in a small plastic drum at 30rpm to spread the dry concentrate over the rock support. Liquid whichcontained 2,000 ppm ferric ion and 1% Nalco #7534 was sprayed onto themixture until all the concentrate was coated onto the rock. The pH ofthe liquid was maintained at 1.8. The amount of liquid used wasestimated to be between 5 and 10 percent of the weight of the coarse oreand concentrate. The 700 g mixture of concentrate on coarse oresubstrates was placed into a 3 inch column. The height of the ore afterbeing placed in the column was approximately 5 inches. Air and liquidwere introduced from the top of the column. The column of concentratecoated on coarse ore substrates was inoculated with about 10 ml ofbacteria at an O.D. of 2.0 or about 8×10⁹ bacteria per ml.

[0181] The bacteria were a mixed culture of Thiobacillus ferrooxidans,which were originally started with ATCC strains #19859 and 33020. Thebacteria were grown in an acidic nutrient solution containing 5 g/lammonium sulfate and 0.83 g/l magnesium sulfate heptahydrate. The pH ofthe solution was maintained in the range of 1.7 to 1.9 by adjustmentwith sulfuric acid (H₂SO₄). The solution also contained iron at 20g/liter in the form of ferric and ferrous sulfate.

[0182] The bacteria were added to the top of the column after the pH wasadjusted to pH 1.8. The liquid, introduced to the top of the columnthroughout the experiment had a pH of 1.8 with 0.2×9 K salts and 2,000ppm ferric ion. The extent of iron oxidation was determined by analysisof the solution eluting off the column minus the iron introduced by the2,000 ppm ferric feed.

[0183] This ore was low in sulfides having a concentration of less than1% of its weight. By making a concentrate on the coarse rock at 20% byweight, the concentration of both the pyrite and gold could be increasedby over tenfold. This increased the rate of biooxidation as seen inFIGS. 6 and 7 over that for the whole ore. Not only did this processexpose more of the pyrite to air and water but it also increased theamount of ferric ion and acid generated per unit volume of ore in thecolumn model for a heap.

[0184]FIG. 6 shows the amount of oxidation as determined by percent ironleached for both the pyrite concentrate of this ore on +0.31 cm coarseore and the whole ore itself. As the graph shows the concentrate processwas biooxidized to about 40% in the first 30 days and over 65% in thefirst 60 days. Whereas the whole ore was only biooxidized to 24% in 84days. The average daily biooxidation rates are shown in FIG. 7. Thehighest average daily rate of the coated concentrate was 1.8% per daycompared to an average daily rate of only 0.5% for the whole ore. AsFIG. 7 illustrates, the coated concentrate sample did not take as longto begin biooxidizing the sample. This means that the coated concentrateprocess is more likely to achieve complete biooxidation in a reasonablyshort time.

[0185] Table 1 below shows the specific data points graphed in FIGS. 6and 7 for the concentrate on coarse ore process and for the whole oreprocess which was done for comparison.

[0186] After 68 days the concentrate coated on coarse ore column wastaken down. The biooxidized material was separated into a plus 180 μmfraction and a minus 180 μm fraction. The weight of the fine materialhad increased from 140 g to 150 g. The total amount of iron removed fromthe system during the 68 days of biooxidation was 21.5 g whichrepresents 46 g of pyrite. The weight of the coarse rock decreased by 54g. This was believed to be due to breakdown of the rock to finermaterial due to the biooxidation process. The total weight afterbiooxidation was 656 g which was 44 g less than the starting material.This fit well with the estimated 46 g of pyrite oxidized. TABLE 1Concentrate Process Whole Ore Process % Fe % Fe # of Days leached %Fe/day # of days Leached % Fe/day 0 0.0 0.00 0 0.0 0.00 9 8.4 0.93 130.2 0.01 16 18.5 1.44 21 2.5 0.29 20 25.5 1.76 28 5.1 0.38 23 31.0 1.8235 8.6 0.50 28 37.5 1.30 42 11.7 0.44 33 41.7 0.84 49 13.8 0.29 37 46.11.10 56 15.9 0.31 43 51.8 0.95 62 18.4 0.42 51 60.7 1.11 70 21.5 0.39 5866.7 0.86 77 23.1 0.23 65 70.9 0.60 84 24.3 0.16

[0187] Two samples of the −180 μm material and one sample of the +180 μmmaterial were leached with cyanide. To leach the samples, bottle rollswere done for 96 hours, the leachant was maintained at 500 ppm cyanide.The +180 μm coarse ore support rock was ground to 95% −75 μm beforedoing the bottle roll. All bottle rolls were done with activated carbonin the leach solution.

[0188] Sulfide analysis of the minus 180 μm fraction after 68 days ofbiooxidation showed the sample still contained 8.8% sulfides which was56% of the starting level. This was a lower percent oxidation thanindicated by the iron leached off during the column experiment. The goldrecovery increased to 84.3% for the high grade (38 ppm) −180 μm fractionand 79.5% for the +180 μm low grade (3 ppm) fraction. This is asubstantial increase from the 45.6% recovery of the unoxidized ore.

EXAMPLE 3

[0189] A sample of 70% minus 75 μm gold ore from a mine in the DominicanRepublic was used to make a sulfide float concentrate. The ore samplewas obtained from the tailing pile at the mine that had already beenleached with cyanide. The ore sample still contained gold values of over2 g per tonne which were occluded within the sulfides and not directlyleachable by cyanide.

[0190] To form the sulfide concentrate, several kilograms of this samplewere further ground to 95% minus 75 μm. The ground sample was thenfloated to form the sulfide concentrate. The flotation was done in smallbatches of 500 g each in a laboratory Wemco flotation cell. Beforeflotation, the ground ore sample was adjusted to a pulp density of 30%.The ore slurry was then mixed with 1.5 Kg/tonne sodium sulfide (Na₂S)for 5 minutes at pH 8.5. Then potassium amyl Xanthate was added as acollector at 100 g/tonne and mixed for 5 minutes. Next 50 g/tonne ofDowfroth D-200 was added and mixed for 5 minutes. Finally, air wasintroduced to produce a sulfide concentrate that contained 17.4% ironand 19.4% sulfide by weight and 14 g of gold per tonne of concentrate. Aplurality of coated substrates were then made by coating 140 g of thesulfide concentrate onto 560 g of +0.31 cm −0.62 cm granite rock. Theconcentrate was added as a dry powder to the granite rock. The mixturewas then rotated in a small plastic drum at 30 rpm to spread the drypyrite over the support material. A liquid which contained 2,000 ppmferric ion and 1% Nalco #7534 agglomeration aid was sprayed on themixture until all the sulfide concentrate was coated onto the wettedgranite rock. The solution was maintained at a pH of 1.8.

[0191] The coarse rock in this case had no iron or gold value. The rock,however, contained a small amount of mineral carbonate which tended tokeep the pH high at first but also provided CO₂ as a carbon source forthe bacteria.

[0192] The 700 g of concentrate coated rock was put into a column. A0.2×9 K salts and 2,000 ppm ferric ion solution having a pH of 1.6 wasintroduced through the top of the column at a flow rate of about 300ml/day. Then the column was inoculated with 10 ml of bacteria as inExample 2. After the pH of the concentrate coated rock substrate wasadjusted to a pH of 1.8, the pH of the influent was set at 1.8. Air wasalso introduced through the top of the column.

[0193]FIG. 8 graphically illustrates the percent of biooxidation asdetermined by the percent of iron leached from the concentrate. Theaverage daily percentage of biooxidation was calculated and is listed inTable 2 and is graphically illustrated in FIG. 9. The percentagebiooxidation was determined by dividing the total iron removed by thetotal iron contained within the concentrate. The rate of biooxidationwas slow to start as the pH was adjusted and the bacteria built up andadapted. However, after about two weeks the rate increased rapidly andreached a maximum after 30 days. By this time almost 50% of the totaliron had been biooxidized. The process continued with a gradual slowdownas the remaining pyrite was consumed. At the end of 64 days nearly 97%of the iron had been biooxidized. Even with the concentrate almostcompletely biooxidized and the rate slowing down near the end of theprocess, the average daily rate was still near 1%/day. After 70 days thebiooxidation was stopped. The biooxidized concentrate was separated intoa plus 180 μm fraction and a minus 180 μm fraction. The weight of thebiooxidized concentrate had decreased from 140 g to 115 g. The totalamount of iron removed from the system during the 70 days ofbiooxidation was 25.9 g which represents 55.5 g of pyrite. The weight ofthe granite rock decreased by 98.8 g. This was believed to be due to abreakdown of the calcium carbonate in the rock by the acid as well asthe breakdown of the rock to finer material. The total weight decreasedby 123.3 g which was 67.8 g more than predicted by biooxidation ofpyrite alone. TABLE 2 Time in Days % Bioox. % Bioox./Day 5 2.590 0.28815 10.270 1.100 22 24.970 2.100 27 37.250 2.450 32 49.700 2.490 3658.610 2.230 42 68.580 1.660 50 82.580 1.750 57 90.870 1.180 64 96.8200.850

[0194] The sample of −180 μm material was leached with 500 ppm cyanidein a bottle roll for 96 hours. The +180 μm granite rock was also leachedwith 500 ppm cyanide to determine how much gold could be stuck to thesupport rock in a process that used barren rock as a supportingsubstrate. Analysis of the −180 μm material showed it still contained9.7% sulfide which indicated only about 50% oxidation.

[0195] Gold extraction was 77% from the −180 μm fraction. This gold wasrecovered from gold ore that had already been leached with cyanide, thusdemonstrating that the process according to the present invention iseven applicable to ores which heretofore have been considered waste. Andwhile any recovery would be an improvement over the process currentlypracticed at the mine, the process according to the present inventionwas able to recover 77% of the gold in what was previously consideredtailings.

[0196] Cyanide leaching of the granite support rock showed that it hadpicked up 0.15 ppm of gold which was 3.4% of the total gold.

EXAMPLE 4

[0197] A sample of gold bearing refractory sulfide ore that had beencrushed to 80% passing 0.62 cm was prepared for testing as support rock.The ore was from the Western States mine located in Nevada, andcontained a high concentration of carbonate minerals in the form oflimestone. The fine material (less than 0.31 cm) was removed in order toallow for good air flow. A four kilogram sample of the +0.31 cm to −0.62cm rock was coated with one kilogram of a gold bearing pyriteconcentrate provided by another mining company. The coating was formedby placing a the coarse ore substrates and dry concentrate into a smallrotating drum and spraying the mixture with a liquid which contained2,000 ppm ferric ion and 1% Nalco #7534 agglomeration aid until all thesulfide concentrate was coated onto the wetted granite rock.

[0198] Iron analysis of both samples showed that the concentratecontained 210 grams of iron and the four kilograms of support rockcontained 42.8 grams of iron.

[0199] The five kilograms of coated ore substrates was placed in a 3inch column. To start the biooxidation process, a solution having a pHof 1.3 and containing 2,000 ppm of ferric ions was passed through thecolumn at about one liter per day. After seven days, the pH of solutionleaving the column was below pH 2.5. At this point the column wasinoculated with 10 ml of a culture of Thiobacillus ferrooxidans bacteria(as in Example 2) and the pH of the feed solution was raised to a pH of1.8. After a total of fifteen days the column was generating acid at apH of 1.7 and an Eh of 700 mV. The progress of the biooxidation processwas followed by measuring the iron leaching off the column ofconcentrate coated nominal 0.62 cm ore. This data was compared with thedata from an experiment using the same concentrate coated on a sample ofbarren rock. The rates of the leaching in both cases are compared ingraph form in FIG. 10. The fact that the Western States experiment wasslightly faster suggests that the coarse ore support rock was alsooxidizing to some extent.

[0200] The Western States column experiment ran for a total of 74 daysand leached a total of 166 grams of iron out of the system or 66% of thetotal iron in both the concentrate and support rock. Most of the ironwas leached from the concentrate, but some came from the support rock.The weight of the concentrate changed from 1,000 grams to 705.8 gramsafter biooxidation. The four kilograms of Western States coarse oresupport rock decreased to 3695.5 grams, which corresponds to a loss of304.5 grams or 7.6% of its weight after biooxidation. The decrease inweight of the coarse ore support rock was due to a combination ofbiooxidation of its pyrite, acid leaching of the carbonate in the ore,and physical abrasion of the ore.

[0201] The 705.8 grams of biooxidized concentrate, which was originallyfrom another mine in Nevada, was tested for gold extraction using acyanide bottle roll test. The gold recovery before biooxidation was 46%.After biooxidation it increased to 86%. This same gold recovery wasachieved by biooxidizing the concentrate to the same extent on thegravel support material.

[0202] The acid consumption of the Western States ore was measuredbefore and after its use as a support rock for biooxidation. The amountof sulfuric acid required to adjust the pH down to 2 before biooxidationwas 31.4 g per 100 g ore. The amount of acid required to adjust the pHdown to 2 after biooxidation was 11 g per 100 g ore. This would meanthat about 20% of the weight of the support rock was acid neutralizedduring the 74 days of biooxidation. This was larger than the 7.6% lossin weight of the support rock. This may be due to a precipitate formingon the rock after biooxidation or sample to sample variation in thepercent limestone.

[0203] Several conclusions can be drawn from this test. First, a low pHbiooxidation process can occur on the surface of a high carbonate ore.Second, with the +0.31 cm to −0.62 cm support material, the process ofneutralization by the pH 1.8 acid was slow enough that the carbonate inthe ore was still not completely removed after 74 days. The process ofslow acid neutralization is beneficial to the bacteria, because theneutralization of the limestone in the ore will provide needed CO₂ forthe biooxidizing bacteria's carbon source. Third, the coarse ore supportwas benefited from the process because smaller nonfloatable sulfides inthe Western State ore were biooxidized.

[0204] Based on the amount of neutralization that occurred in about 2months of the +0.31 cm to −0.62 cm coarse ore support, a +0.62 cm to−1.9 cm coarse ore support rock would be best for a full scale process.With the larger coarse ore support, it will take 90 to 120 days in aheap biooxidation process to make the best use of the limestoneneutralization and to biooxidize the smaller floatable sulfides in thecoarse ore support rock. The time it takes to biooxidize the coating ofsulfide spread on the outside of the coarse ore support is generallyless than 90 days. Therefore, the coarse ore support may be used severaltimes before it is ground up and floated to make a pyrite concentratefor biooxidation on the surface of a coarse ore support rock.

[0205] Prior to biooxidation, two attempts were made to produce aconcentrate by flotation of the Western States ore. One method used onlyxanthate and produced only a small recovery of gold (less than 12%) intothe pyrite concentrate. The tail from this flotation still contained 4.0g Au/tonne. Extraction of the flotation tail with cyanide only recovered17% of the gold remaining in the tail.

[0206] A second attempt at flotation used both kerosene to float off acarbon concentrate followed by xanthate to produce a pyrite concentrate.The combined weight of these concentrates accounted for 18 weight % ofthe ore, which was double the 7.4 weight % concentrate produced usingonly xanthate. The combined gold recovery for both concentratesincreased to 53.8% of the gold. The tail from this flotation decreasedto 2.12 g/tonne in gold. Extraction of the tail with cyanide recoveredonly 34.5% of the gold remaining in the tail after flotation of bothconcentrates.

[0207] The third attempt at flotation was done with the Western Statesore after it had been used as a support rock for biooxidation in thepresent example. The +0.31 cm to −0.62 cm ore substrates were ground to−75 μm and then floated using xanthate as a collector. This formed apyrite concentrate of 33.4 g Au/tonne and 7.9% of the original oreweight. The tail from this flotation contained 1.09 g Au/tonne. Therecovery of the gold into the pyrite concentrate was 72.4%. Cyanideextraction of the 1.09 g/tonne tail recovered 48.7% of the gold toproduce a final tail of 0.56 g/tonne.

[0208] The 33.4 g Au/tonne pyrite concentrate was biooxidized in a shakeflask experiment. After biooxidation the cyanide extraction hadincreased to 99% gold recovery. This result showed that this concentratewas gold containing pyrite that could be biooxidized along with otherconcentrate in the coated substrate process.

[0209] As can be seen from the flotation results contained in Table 3below, by floating the Western States ore after it was used as a supportmaterial for biooxidation, a high grade pyrite concentrate was moreeasily produced, and the flotation tail was less refractory to cyanideextraction. This may have been due to a chemical change to the pyriteduring the 74 days in the high ferric and low pH conditions ofbiooxidation. Alternatively, the nonfloating sulfides may have been madeless refractory by a combination of ferric and bacterial oxidation.TABLE 3 FLOTATION RESULTS 1st Pyrite 3rd after Float 2nd float bioox.float grinding −75 μm −75 μm −75 μm reagents for Xanthate Kerosene NaS,CuSO4 flotation Dowfroth NaSiO3 Xanthate Xanthate Dowfroth Dowfroth Wt.% of 7.4% 3.2% 7.9% pyrite conc. Wt. % of — 14.8% — carbon conc. Totalwt. % 7.4% 18.0% 7.9% of conc. Grade of 6.4 g/t 26.4 g/t 33.4 g/t conc.% gold in 11.3% 53.8% 72.4% conc. Gold in tail 4.0 g/t 2.12 g/t 1.09 g/tbefore CN Gold in tail 3.32 g/t 1.39 g/t 0.56 g/t after CN Gold recovery17.2% 34.5% 48.7% from leaching tail by CN Combined 26.4% 69.2% 85.4%total recovery Head grade of 4.18 g/t 3.77 g/t 3.64 g/t sample tested

[0210] Although the invention has been described with reference topreferred embodiments and specific examples, it will readily beappreciated by those of ordinary skill in the art that manymodifications and adaptations of the invention are possible withoutdeparture from the spirit and scope of the invention as claimedhereinafter. For example, while the processes according to the presentinvention have been described in terms of recovering gold fromrefractory sulfide or refractory carbonaceous sulfide ores, theprocesses are equally applicable to other precious metals found in theseores such as silver and platinum. Similarly, the process according tothe present invention may, as one skilled in the art would readilyrecognize, be used to biooxidize sulfide concentrates from metal sulfideores such as chalcopyrite and sphalorite.

I claim:
 1. A method of biotreating and recovering metal values frommetal-bearing refractory sulfide ore using a nonstirred surfacebioreactor, said process comprising the steps of: a. producing aconcentrate of metal sulfide particles from the refractory sulfide ore,b. coating the surface of a plurality of coarse substrates having aparticle size of greater than about 0.3 cm with the metal sulfideparticles to be biotreated and thereby forming a plurality of coatedcoarse substrates, said metal sulfide particles to be biotreated havinga particle size of less than about 250 μm. c. forming a nonstirredsurface reactor by stacking said plurality of coated coarse substratesinto a heap or placing said plurality of coated coarse substrates into atank, said reactor having a void volume greater than or equal to about25% and a surface area of greater than or equal to 100 square meters percubic meter of reactor space; d. biooxidizing the metal sulfideparticles on the plurality of coarse substrates; e. contacting thebiooxidized metal sulfide particles with a metal lixiviant to therebydissolve metal values from the biooxidized metal sulfide particles; andf. recovering precious metal values from the lixiviant.
 2. A methodaccording to claim 1, wherein said plurality of coarse substrates arerock, and said rock is selected from the group consisting of lava rock,gravel, and barren rock containing carbonate minerals.
 3. A methodaccording to claim 1, wherein no more than 5% by weight of said coarsesubstrates are less than 0.3 cm.
 4. A method according to claim 1,wherein the surface area of the reactor per cubic meter of reactor spaceis greater than or equal to 500 square meters per cubic meter ofreactor.
 5. A method of bioremediating contaminated soils comprising thesteps of: a. coating the surface of a plurality of coarse substrateshaving a particle size of greater than about 0.3 cm with contaminatedsoil to be biotreated and thereby forming a plurality of coated coarsesubstrates, said contaminated soil to be biotreated having a particlesize of less than about 250 μm; b. forming a nonstirred surface reactorby stacking said plurality of coated coarse substrates into a heap orplacing said plurality of coated coarse substrates into a tank, saidreactor having a void volume greater than or equal to about 25% and asurface area of greater than or equal 100 square meters per cubic meterof reactor space; c. biotreating said contaminated soil on the surfaceof the plurality of coarse substrates until said undesired organiccompound in the contaminated soil is degraded to a desiredconcentration.
 6. A method according to claim 5, wherein said pluralityof coarse substrates are rock, and said rock is selected from the groupconsisting of lava rock, gravel, and barren rock containing carbonateminerals.
 7. A method according to claim 5, wherein no more than 5% byweight of said coarse substrates are less than 0.3 cm.
 8. A methodaccording to claim 5, wherein the surface area of the reactor per cubicmeter of reactor space is greater than or equal to 500 square meters percubic meter of reactor.
 9. A method of biotreating a solid material toremove an undesired compound using a nonstirred surface bioreactor, saidprocess comprising the steps of: a. coating the surface of a pluralityof coarse substrates having a particle size of greater than about 0.3 cmwith a solid material to be biotreated and thereby forming a pluralityof coated coarse substrates, said solid material to be biotreated havinga particle size of less than about 250 μm and containing an undesiredcompound; b. forming a nonstirred surface reactor by stacking saidplurality of coated coarse substrates into a heap or placing saidplurality of coated coarse substrates into a tank, said reactor having avoid volume greater than or equal to about 25% and a surface area ofgreater than or equal 100 square meters per cubic meter of reactorspace; c. inoculating said reactor with a microorganism capable ofdegrading the undesired compound in said solid material to thereby forma nonstirred surface bioreactor; and d. biotreating said solid materialin said bioreactor until said undesired compound in said solid materialis degraded to a desired concentration.
 10. A method according to claim9, wherein said plurality of coarse substrates are rock, and said rockis selected from the group consisting of lava rock, gravel, and barrenrock containing carbonate minerals.
 11. A method according to claim 9,wherein no more than 5% by weight of said coarse substrates are lessthan 0.3 cm.
 12. A method according to claim 9, wherein the surface areaof the reactor per cubic meter of reactor space is greater than or equalto 500 square meters per cubic meter of reactor.